Then having clearly justified himself in every particular, and proved that he had spoken nothing from anger or malevolence, but had preserved his affection towards them unimpaired, he again establishes this same point by concluding his discourse with a prayer teeming with a thousand blessings, in these words;
Ver. 18. “The grace of our Lord Jesus Christ be with your spirit, brethren. Amen.”
By this last word he has sealed all that preceded it. He says not merely, “with you,” as elsewhere, but, “with your spirit,” thus withdrawing them from carnal things, and displaying throughout the beneficence of God, and reminding them of the grace which they enjoyed, whereby he was able to recall them from all their judaizing errors. For to have received the Spirit came not of the poverty of the Law, but of the righteousness which is by Faith, and to preserve it when obtained came not from Circumcision but from Grace. On this account he concluded his exhortation with a prayer, reminding them of grace and the Spirit, and at the same time addressing them as brethren, and supplicating God that they might continue to enjoy these blessings, thus providing for them a twofold security. For both prayer and teaching, tended to the same thing and together became to them as a double wall. For teaching, reminding them of what benefits they enjoyed, the rather kept them in the doctrine of the Church; and prayer, invoking grace, and exhorting to an enduring constancy, permitted not the Spirit to depart from them. And He abiding in them, all the error of such doctrines as they held was shaken off like dust.
Source: Commentary on Galatians (New Advent)