“Sanctification” is a key concept in New Testament Christianity, but it is a concept that many have misunderstood. The word “sanctify” means “to set apart, make holy for a sacred purpose.” In one sense, this is a continual growth process for Christians, as Paul wrote to a group of Christians, “This is the will of God, your sanctification” (1 Thess. 4:3). Christians must continually endeavor to keep themselves set apart from the world (cf. Rom. 12:1-2).
But, is there a point at which a person who was not sanctified becomes, right then, sanctified? When a person who was not holy becomes holy? The Bible answers that question more than once. Christ “loved the church and gave Himself for her, that He might sanctify her” (Eph. 5:26). When is someone sanctified? There is a participle verb that follows the main verb “sanctify” 
that tells us when it happened – “having cleansed her by the washing of water with the Word.” One is “sanctified” by being baptized (participle, being cleansed by water) – set apart, at that moment, and made holy in the sight of God. Then, it is the Christian’s responsibility to steadfastly remain in that sanctified state (1 Thess. 5:23; Heb. 2:11; 1 Pet. 1:15-16).