On the day of Pentecost at Jerusalem after having received the Comforter, the Holy Ghost, which was Jesus, 2 Corinthians 3:17. Matthew 28:19 by proper interpretation becomes Acts 2:38 by the power of the Holy Ghost, Romans 15:13.
"Nevertheless when one turns to the Lord, the veil is taken away. Now the Lord is the Spirit; and where the Spirit of the Lord is, there is liberty. But we all, with unveiled face, beholding as in a mirror the glory of the Lord, are being transformed into the same image from glory to glory, just as by the Spirit of the Lord" (II Corinthians 3:16-18).
The basic arguement is that God is composed of only one being, whom we know as Jesus Christ. The Jews knew him as Yahweh or El Shaddai and Christians received Him as the Holy Spirit.
Long ago David wrote, "The sum of Your word is truth, and every one of Your righteous ordinances is everlasting" (Psalm 119:160). Truth is established by viewing the Bible as a whole and not selecting a small section. If the phrase in II Corinthians 3:17, "Now the Lord is the Spirit" was meant to be understood that the Lord Jesus and the Holy Spirit are one and the same being, then we would expect to find support for this throughout the Bible. The fact, though, is that it doesn't exist.
At the baptism of Jesus we find the Father, the Son, and the Holy Spirit all mentioned. "When He had been baptized, Jesus came up immediately from the water; and behold, the heavens were opened to Him, and He saw the Spirit of God descending like a dove and alighting upon Him. And suddenly a voice came from heaven, saying, "This is My beloved Son, in whom I am well pleased."" (Matthew 3:16-17). There are three beings mentioned in three locations: Jesus coming up out of the water, the Spirit descending like a dove and a voice proclaiming from Heaven that Jesus was His beloved Son.
Paul stated, "There is one body and one Spirit, just as you were called in one hope of your calling; one Lord, one faith, one baptism; one God and Father of all, who is above all, and through all, and in you all" (Ephesians 4:4-6). This is a list of unique things. If you claim that the one Spirit is also the one Lord, then why could I not also claim that the one body (the church) is the one Spirit and the one Father? No, each of the things listed are different and unique.
But the greatest difficulty is found in John 15:26, "But when the Helper comes, whom I shall send to you from the Father, the Spirit of truth who proceeds from the Father, He will testify of Me." Jesus said he would send the Holy Spirit from the Father. Hence, Jesus could not be the Spirit since Jesus was sending the Spirit to the apostles. Notice that Jesus speaks of the Spirit in the third person. He didn't say "I will testify of Me" he said "He will testify of Me."
Look again at II Corinthians 3:16-18. Is there any indication in the words that the Spirit referenced is the Holy Spirit? Readers of the Bible sometimes forget that "God is Spirit, and those who worship Him must worship in spirit and truth" (John 4:24). If we accept that Jesus is God, then that implies He is Spirit. Speaking of Jesus, Paul said, "And so it is written, "The first man Adam became a living being." The last Adam became a life-giving spirit" (I Corinthians 15:45).
There is another way to read this passage as well. The Spirit in II Corinthians 3:17 can be seen as the Spirit of the Mosical Law. For example, "And I fell at his feet to worship him. But he said to me, "See that you do not do that! I am your fellow servant, and of your brethren who have the testimony of Jesus. Worship God! For the testimony of Jesus is the spirit of prophecy"" (Revelation 19:10). Here the spirit of prophecy is the intent, the goal, or the driving force behind prophecy. In this case, the spirit of prophecy is the testimony (or witness) of Jesus. This is how "Spirit" was being used in II Corinthians 3. "You are our epistle written in our hearts, known and read by all men; clearly you are an epistle of Christ, ministered by us, written not with ink but by the Spirit of the living God, not on tablets of stone but on tablets of flesh, that is, of the heart. And we have such trust through Christ toward God. Not that we are sufficient of ourselves to think of anything as being from ourselves, but our sufficiency is from God, who also made us sufficient as ministers of the new covenant, not of the letter but of the Spirit; for the letter kills, but the Spirit gives life. But if the ministry of death, written and engraved on stones, was glorious, so that the children of Israel could not look steadily at the face of Moses because of the glory of his countenance, which glory was passing away, how will the ministry of the Spirit not be more glorious?" (2 Corinthians 3:2-8). The spirit of the message of Jesus Christ brought life to the hearers of that message. It was delivered by the Holy Spirit to the apostles (John 14:26) and, simultaneously, the spirit of the message (its core or heart) was Jesus Christ. That message brought to Christians liberty to speak boldly "because the veil is taken away in Christ" (II Corinthians 3:14).
Basically, II Corinthians 3 is an amplification of what Jesus stated, "It is the Spirit who gives life; the flesh profits nothing. The words that I speak to you are spirit, and they are life" (John 6:63). The Spirit delivered the spirit of the message and that message is Jesus. "Then Philip opened his mouth, and beginning at this Scripture, preached Jesus to him" (Acts 8:35).
The interpretation you gave to the phrase is not a necessary conclusion. You assumed that it meant that Jesus and the Holy Spirit are one and the same, yet that assumption leads to contradictions with many other passages in the Bible. There are other ways to understand that same phrase that does not lead to such a contradiction.