Because God is Spirit, he has given through Christ the law of the Spirit, which persuades us to believe in invisible things which our reasoning understands spiritually. This law gives liberty because it demands only faith, and because it believes what it does not see, we are able to be rescued from our condition.
The same, then, is the Lord, who is the Spirit of the Lord; that is, he called the Spirit of the Lord, Lord, just as also the apostle says: “Now the Lord is a spirit, and where the Spirit of the Lord is, there is liberty.” You have then, the Lord called also the Holy Spirit; for the Holy Spirit and the Son are not one person but one substance.
Just like children, so are the Jews also under a tutor. The law is our tutor; a tutor brings us to the master; Christ is our only master.… A tutor is feared, the master points out the way to salvation. Fear brings us to liberty, liberty to faith, faith to love, love obtains adoption, adoption an inheritance. Therefore, where there is faith, there is freedom, for a slave acts in fear, a free man through faith. The one is under the letter, the other under grace; the one in slavery, the other in the spirit; for “Where the Spirit of the Lord is, there is freedom.”.
"Now the Spirit is the Lord." This too is Lord, he says. And that you may know that he is speaking of the Paraclete, he added,
"And where the Spirit of the Lord is, there is liberty."
For surely you will not assert, that he says, 'And where the Lord of the Lord is.' "Liberty," he said, with reference to the former bondage. Then, that you may not think that he is speaking of a time to come, he says...
The person who has been blessed with the Spirit of the Lord has been set free from the condemnation of the law, for the spiritual gifts are given their power through the Spirit. Moreover, the gift is given freely to those who are ready to receive it.
If Paul had wanted to say that the Lord is a spirit, he would have left the article the out. Compare John [4:24], where in speaking to the Samaritan woman, Jesus says that God is a spirit, meaning that he does not have a body. But in this case Paul puts the article in, which proves that he is not saying that the Lord is a spirit but rather that the Spirit is Lord.
Paul shows here that the Spirit and God are equal. Moses turned his eyes toward God; we turn ours toward the Holy Spirit. Paul would hardly have said that what the Spirit reveals is greater than what Moses saw if the Spirit were merely a creature and not God himself.
[AD 384] Ambrosiaster on 2 Corinthians 3:17