Paul made it clear that he was staying at Ephesus because he had found hearts there who were thirsting for the grace of God, and in them he could quickly instill the mystery of Christ. But because the devil is always restless and hostile to those who long for God, he adds that his enemies there are many. For the more they sought after the faith, the more there were adversaries who contradicted them and fought against the teaching of the Lord.
As we should expect, he tells them all exactly, informing them as friends, For this too is a mark of friendship to say the reason why he was not with them, why he delayed, and where he was staying.
"For a great door and effectual is opened unto me, and there are many adversaries."
Now it was "great," how could there be "adversaries?" Why on this very account the adversaries were many, because men's faith was great; because the entrance was great and wide. But what means, "A great door?" There are many prepared to receive the faith, many ready to approach and be converted. There is a spacious entrance for me, things being now come to that point that the mind of those approaching is at its prime for the obedience of the faith. On this account, vehement was the blast of the breath of the devil, because he saw many turning away from him.
You see then on both accounts it was needful for him to stay; both because the gain was abundant, and because the struggle was great.
And herewith also he cheered them up, namely, by saying, that henceforth the word works every where and springs up readily. And if there be many who plot against it, this also is a sign of the advance of the Gospel. For at no time doth that evil demon wax fierce, except on seeing his goods made spoil of abundantly.
Let us then, when we desire to effect any thing great and noble, not regard this, the greatness of the labor which it brings, but let us rather look to the gain. Mark, for instance, Paul, not therefore lingering, not therefore shrinking back, because "there were many adversaries;" but because "there was a great door," pressing on and persevering. Yea, and as I was saying, this was a sign that the devil was being stripped, for it is not, depend on it, by little and mean achievements that men provoke that evil monster to wrath. And so when thou seest a righteous man performing great and excellent deeds, yet suffering innumerable ills, marvel not; on the contrary, one might well marvel, if the devil receiving so many blows were to keep quiet and bear the wounds meekly. Even as you ought not to be surprised were a serpent, continually goaded, to grow fierce and spring on the person that goaded it. Now no serpent steals on you so fierce as the devil, leaping up against all; and, like a scorpion with its sting raised, he raises himself upright. Let not this then disturb you: since of course he that returns from war and victory and slaughter must needs be bloody, and oftentimes also have received wounds. Do thou, then, for thy part, when thou seest any one doing alms and performing numberless other good works and so curtailing the power of the devil, and then falling into temptations and perils; be not troubled thereupon. This is the very reason why he fell into temptations, because he mightily smote the devil.
[AD 384] Ambrosiaster on 1 Corinthians 16:9