The grace of the Lord Jesus Christ, and the love of God, and the communion of the Holy Ghost, be with you all. Amen" 2 Corinthians 13:14. This classic greeting of Paul is a clear demonstration of his Trinitarian belief. Why else would he mention all three if he did not mean the trinity?

Answer:

It must be first noted that this is not a classical greeting of Paul. The classical and habitual greeting of Paul (and others) can be found in the introduction of the epistles where the consistent greeting is along these lines "Grace be unto you, and peace, from God our Father, and from the Lord Jesus Christ."
1Corinthians 1:3 (see also Romans 1:7; 2 Corinthians 1:2; Galatians 1:3; Ephesians 1:2; Philippians 1:2; Colossians 1:2; 1 Thessalonians 1:1; 2 Thessalonians 1:2; 1 Timothy 1:2; 2 Timothy 1:2; Titus 1:4; Philemon 1:3; 1 Peter 1:2; 2 John 1:3).

The text speaks of:
1) The grace of the Lord Jesus Christ.
2) The love of God.
3) The communion of the Holy Ghost.

In this verse the title God is used only once and it is used in reference to a specific person. Obviously, it is God the Father whose love is spoken of in this verse, and, according to John 3:16, He loves us so much that He gave His only begotten Son to give grace to us that we can be drawn into communion
with them both through the Spirit. Ephesians 2:18 "For through him we both have access by one Spirit unto the Father."

Jesus said, "If a man love me, he will keep my words: and my Father will love him, and we will come unto him, and make our abode with him" John 14:23.
How will the Father and the Son be with us and in us? "Do you not know that you are God's temple and that God's Spirit dwells in you?" 1Corinthians 3:16

Please observe that Paul did not say we would commune with the Holy Ghost, but he speaks of "the communion of the Holy Ghost." What does he mean by this? When Paul spoke of the communion of the Holy Ghost, he was showing how our communion would be. It would be by the means of the Holy Spirit that we would commune or have fellowship with God and Jesus Christ.

1 John 1:3 states, "That which we have seen and heard declare we unto you, that ye also may have fellowship with us: and truly our fellowship is with the Father, and with his Son Jesus Christ."

According to John, we have fellowship with the Father and with his Son, through the Holy Spirit. Paul wanted this experience to remain with the Christians in Corinth and also with us today, an experience of true fellowship with the Father and with his Son through the Holy Spirit.