While rejecting the doctrine of entire sanctification in this life, orthodox Christians look forward to the day, guaranteed by our Lord, when our sanctification will be perfect.
All believers will be kept blameless until Christ’s return. At that time, they will be perfectly sanctified forever in glory:Now may the God of peace himself sanctify you completely, and may your whole spirit and soul and body be kept blameless at the coming of our Lord Jesus Christ. He who calls you is faithful; he will surely do it. —1 Thessalonians 5:23–24
This concluding doxology provides additional instruction on the eternal security of the believer. The eternal duration of salvation is a guaranteed reality that rests not upon the shaky faithfulness of believers but upon the unwavering steadfastness of God Himself. It is God’s faithfulness that will bring His eternal purposes for His elect to full completion. God, who calls His elect to Himself in conversion, also calls them to sanctification and glorification, and He never fails to bring this ongoing salvation to pass. Charles A. Wanamaker writes, “The present tense of the participle kalon (“the one calling”) stresses that God does not merely call Christians once and then leave them on their own. Instead God continues to call the followers of Christ to salvation. Mention of God’s faithfulness by Paul is intended to assure the Thessalonians that God will not reject them or withdraw the call directed to them.” This is to say, God keeps all believers secure until He brings them to a future state of completed sanctification, otherwise known as glorification. This will be a future reality.
—Steve Lawson, Foundations of Grace (Reformation Trust, 2006), 437–438.