May the God of peace himself sanctify you completely. May your whole spirit, soul, and body be preserved blameless at the coming of our Lord Jesus Christ.
Sanctify you wholly ...  Despite the "setting apart" unto God that takes place in one's conversion (faith, repentance and baptism) into Christ, the sanctification in this verse refers to the continuation of the sanctification that is begun at the beginning of the Christian life.  The word "wholly" indicates that it is a perfected or completed state of sanctification that Paul desired the Thessalonians to possess.