From Second Corinthians, by Thomas D. Stegman, SJ, commenting on II Corinthians 13:11-13
At long last, Paul brings the letter to a formal conclusion as he offers a solemn benediction: The grace of the Lord Jesus Christ and the love of God and the fellowship of the holy Spirit be with all of you.
This closing blessing is particularly striking when compared with Paul’s other concluding blessings. Typically, he ends his letters thus: “The grace of the (or our) Lord Jesus (Christ) be with you.” The additions to this typical benediction in verse 13 are important on the level of theology and the level of Christian life. In terms of theology, the additions serve to remind the Corinthians of the chief theological concerns of the letter. Central to this final blessing is “the love of God.” God’s love for us is such that he holds nothing back from us in order that we might have the fullness of life.
As we just saw at the end of verse 11, God’s love has been manifested in two supreme ways, both of which are signified here. First, God has sent his Son to save us from sin and death (5:18–21). Hence Paul refers to “the grace of our Lord Jesus Christ,” the same phrase he employed in 8:9 to signify Jesus’ self-giving love in becoming human and offering his life on the cross in obedience to the Father’s will. Second, God has bestowed in our hearts the gift of the Holy Spirit (1:22; 3:3) to purify and sanctify us, to make us his holy people (6:16). Hence Paul adds the reference to the Spirit.
In addition to its theological import, the solemn final blessing functions as a bulwark for life in the Church. The phrase “the fellowship (koinōnia) of the Holy Spirit” is, as one commentator aptly notes, “powerfully ambiguous.” It refers to the gift of the Spirit in us whereby each one of us is, literally, in close communion with God.