15 And he said unto them, Go ye into all the world, and preach the gospel to every creature.
[AD 165] Justin Martyr on Mark 16:15
For from Jerusalem there went out into the world, men, twelve in number, and these illiterate, of no ability in speaking: but by the power of God they proclaimed to every race of men that they were sent by Christ to teach to all the word of God.

[AD 220] Tertullian on Mark 16:15
Then in this same way the apostles went out to found churches in every city possible. It is from these apostolic churches that all the subsequent churches, one after the other, derived the rule of faith and the seeds of doctrine. Even to today they continue to derive from the apostles that which is necessary in order that they be churches. Indeed, it is for this reason only that they are able to deem themselves as apostolic, as being the offspring of apostolic churches. As in science, every genus reverts to its original for its classification, so with the apostolic church. However many or great these churches may be, they comprise but one primitive church, founded by the apostles, from which they all spring. In this way all are primitive. All are apostolic. They all are one, by means of their unbroken unity, peaceful communion, title of descent, and bond of hospitality. These are privileges that no other rule directs than the one tradition of the same mystery.

[AD 220] Tertullian on Mark 16:15
Again, in the Pslams, David says: "Bring to God, ye countries of the nations"-undoubtedly because "unto every land" the preaching of the apostles had to "go out" -"bring to God fame and honour; bring to God the sacrifices of His name: take up victims and enter into His courts.

[AD 258] Novatian on Mark 16:15
He willed that the apostles as spiritual progenitors of the new humanity would be sent by his Son into the entire world, so that all human sufferers might come to the knowledge of their creator. Insofar as any choose to follow him, they have One whom they now address in their prayers as Father, instead of God. His providence has run and at present runs its course not only among individuals but also through whole cities and states, whose overthrow he predicted by the words of the prophets. His providence indeed runs its course through the whole cosmos itself.

[AD 430] Augustine of Hippo on Mark 16:15
The command to the apostles to be witnesses to him in Jerusalem, Judea, Samaria, and even to the uttermost parts of the earth was not addressed exclusively to those to whom it was immediately spoken. They alone would not be the only ones who would carry such an enormous task to completion. Similarly he seems to be speaking to the apostles very personally when he says: “Behold I am with you even to the end of the world,” yet who does not know that he made this promise to the universal church which will last from now even to the consummation of the world by successive births and deaths?

[AD 604] Gregory the Dialogist on Mark 16:15
My brothers, was the holy Gospel to be preached to insensible things or to brute animals, that it is said to the disciples concerning it: "Preach to every creature"? But by the name of every creature, man is signified. For there are stones, but they neither live nor feel. There are plants and trees; they live indeed, but they do not feel. They live, I say, not through a soul, but through their verdure, because Paul also says: "Foolish one, what you sow is not brought to life unless it first dies." Therefore that lives which dies, so that it may be brought to life. Stones therefore exist, but they do not live. Trees exist and live, but they do not feel. Brute animals exist, live, and feel, but they do not discern. Angels exist, live, feel, and discern. But man has something of every creature. For he has existence in common with stones, life with trees, sensation with animals, understanding with angels. If therefore man has something in common with every creature, in a certain sense every creature is man. Therefore the Gospel is preached to every creature when it is preached to man alone, because he is taught for whose sake all things were created on earth, and from whom all things are not alien through a certain likeness.

By the name of every creature, every nation of the Gentiles can also be designated. For before it had been said: "Do not go into the way of the Gentiles." But now it is said, "Preach to every creature," so that the preaching of the apostles, first rejected by Judea, might then become a help to us, when that proud people had rejected it as a testimony to their own condemnation.

But when Truth sends the disciples to preach, what else does He do in the world but scatter grains of seed? And He sends a few grains in the sowing, so that He may receive the fruits of many harvests from our faith. For so great a harvest of the faithful would never have arisen in the whole world, if those chosen grains of preachers had not come from the Lord's hand upon the rational earth.

[AD 604] Gregory the Dialogist on Mark 16:14-18
(ubi sup.) We should observe that Luke says in the Acts, As he was eating with them he commanded that they should not depart from Jerusalem, and shortly afterwards, while they beheld he was taken up. For He ate, and then ascended, that by the act of eating, the truth of the flesh might be declared; wherefore it is also here said, that he appeared to them for the last time as they sat at meat.

(ubi sup.) Another reason also why our Lord rebuked His disciples, when He left them as to His bodily presence, was, that the words which He spoke on leaving them might remain more deeply impressed upon the hearts of His hearers.

(ubi sup.) After rebuking the hardness of their hearts, let us hear the words of advice which He speaks. For it goes on: Go ye into all the world, and preach the Gospel to every creature. Every man must be understood by every creature; for man partakes something of every creature; he has existence as have stones, life as trees, feeling as animals, understanding as have Angels. For the Gospel is preached to every creature, because he is taught by it, for whose sake all are created, whom all things are in some way like, and from whom therefore they are not alien. By the name of every creature also every nation of the Gentiles may be meant. For it had been said before, Go not into the way of the Gentiles. (Matt. 10:5) But now it is said, Preach the Gospel to every creature, so that the preaching of the Apostles which was thrust aside by Judæa, might be an assistance to us, since Judæa had haughtily rejected it, thus witnessing to her own damnation.

(ubi sup.) But perhaps some one may say in himself, I have already believed, I shall be saved. He says what is true, if he keeps his faith by works; for that is a true faith, which does not contradict by its deeds what it says in words. There follows: But he that believeth not shall be damned.

(ubi sup.) Are we then without faith because we cannot do these signs? Nay, but these things were necessary in the beginning of the Church, for the faith of believers was to be nourished by miracles, that it might increase. Thus we also, when we plant groves, pour water upon them, until we see that they have grown strong in the earth; but when once they have firmly fixed their roots, we leave off irrigating them. These signs and miracles have other things which we ought to consider more minutely. For Holy Church does every day in spirit what then the Apostles did in body; for when her Priests by the grace of exorcism lay their hands on believers, and forbid the evil spirits to dwell in their minds, what do they, but cast out devils? And the faithful who have left earthly words, and whose tongues sound forth the Holy Mysteries, speak a new language; they who by their good warnings take away evil from the hearts of others, take up serpents; and when they are hearing words of pestilent persuasion, without being at all drawn aside to evil doing, they drink a deadly thing, but it will never hurt them; whenever they see their neighbours growing weak in good works, and by their good example strengthen their life, they lay their hands on the sick, that they may recover. And all these miracles are greater in proportion as they are spiritual, and by them souls and not bodies are raised.

[AD 735] Bede on Mark 16:15
And he said to them: Go into the entire world and preach the Gospel to every creature. He who believes and is baptized will be saved; but he who does not believe will be condemned. Therefore, when you preach that whoever does not believe will be condemned, since he certainly does not believe what he has not seen, should not they themselves first be reproved, because they did not believe those to whom the Lord appeared before they had seen the Lord? But when he said to them, Go into the entire world, preach the Gospel to every creature, surely the holy Gospel was not meant to be preached to senseless things or brute animals, so that it would be said to the disciples, Preach to every creature? But under the name of every creature, every nation of the Gentiles can be designated. It had been previously said: Do not go into the way of the Gentiles; but now it is said, Preach to every creature, so that the preaching of the apostles, first repelled by Judea, would then assist us, since Judea had proudly repelled it to its own damnation. He who believes and is baptized will be saved; but he who does not believe will be condemned. Perhaps each one says to himself: I have already believed, I will be saved. He says the truth if he holds faith by works. For true faith is that which does not contradict in behavior what it says in words. From this comes what Paul says about certain false believers: They confess to know God, but in works they deny him (Titus 1). Here John says: He who says he knows God and does not keep his commandments is a liar (1 John 2). But when it is said, He who does not believe will be condemned, what do we say about infants who cannot yet believe because of their age? For with regard to adults, there is no question. Therefore, in the Church, infants believe through others on behalf of the Savior, just as they have derived the sins from others, which are remitted to them in baptism.

[AD 735] Bede on Mark 16:14-18
(ubi sup.) What shall we say here about infants, who by reason of their age cannot yet believe; for as to older persons there is no question. In the Church then of our Saviour children believe by others, as also they drew from others the sins which are remitted to them in baptism. It goes on: And these signs shall follow them that believe; In my name shall they cast out devils; they shall speak with new tongues; they shall take up serpents.

[AD 1107] Theophylact of Ohrid on Mark 16:14-18
Or else; to every creature, that is, whether believing or unbelieving. It goes on: He that believeth and is baptized shall be saved. For it is not enough to believe, for he who believeth and is not baptized, but is a catechumen, has not yet attained to perfect salvation.

That is, they shall scatter before them serpents, whether intellectual or sensible, as it is said, Ye shall tread upon serpents and scorpions, which is understood spiritually. But it may also mean sensible serpents, as when Paul received no hurt from the viper. There follows: And if they drink any deadly thing, it shall not hurt them. (Luke 10:19) We read of many such cases in history, for many persons have drank poison unhurt, by guarding themselves with the sign of Christ. It goes on: They shall lay hands on the sick, and they shall recorer.

[AD 1274] Glossa Ordinaria on Mark 16:14-18
(non occ.) Mark, when about to finish his Gospel, relates the last appearance of our Lord to His disciples after His resurrection, saying,1 For the last time he appeared unto the eleven as they sat at meat. (Acts 1:4, 9)

[AD 1274] Pseudo-Jerome on Mark 16:14-18
But in a mystic sense we may understand that faith here labours, leading the active life, but there it reigns secure in the contemplative vision. Here we see His face through a glass, there we shall see the truth face to face, wherefore He was shown to them as they were walking, that is, labouring, in another form. And when it was told, the disciples did not believe, because they saw, like Moses, that which was not enough for them, for he said, show me thyself; (Exod. 33:18. Sept.) forgetting his flesh, he prays in this life for that which we hope for in the life to come.

But He appeared when all the eleven were together, that all might be witnesses, and relate to all men what they had seen and heard in common. It goes on: And upbraided them with their unbelief and hardness of heart, because they believed not them who had seen him after his resurrection.

But He rebukes their want of faith, that faith might take its place; He rebukes the hardness of their stony heart, that the fleshy heart, full of love, might take its place.