The epistle is rapidly drawn to a close in these final verses: v10 is a summary of the gospel, highlighting the call to love others, and v11 is a summary of Paul’s authority emphasizing his personal authorship of the letter and his passion for them expressed through his “large letters.”
Then Paul summarizes the problem of the Judaizers, declaring that they’re evading the gospel by avoiding persecution on account of Christ’s cross, and they’re attempting to boast in the Law by counting their converts to circumcision. Verse 14 here connects back to 6:4 – we can only boast in Christ’s work in us, not the Law which convicts us, nor by comparing ourselves to others which God doesn’t do! To summarize the Judaizer problem, v15 states that circumcision means nothing now; it’s the new creation that counts. Connecting this back to 3:27 and 2 Cor. 5:17, Paul is probably hinting at the fact that Baptism has replaced circumcision as the method of entry into God’s people.
Paul wishes peace and mercy on them, the Israel of God. This is also a rebuke against Judaizers who think Israel is about circumcision and Law, when in fact the Israel of God is the Church. The “marks of Christ” are probably literal scars from Paul’s misadventures, and parallel a slave’s marks identifying their master, or Pagan priests’ marks identifying their gods.
Verse 18 closes with a standard formal goodbye with no personal greetings at all. But at least the benedictory words involve grace – the essence of the gospel! This is a bittersweet ending for a letter.