Qadash is the Hebrew word for “to sanctify” in the Old Testament, and it means to dedicate, hallow, consecrate, purify, and be or keep “holy.” Hagiasmos is the Greek word for “sanctification” which can be found in the New Testament and it means separation or setting apart. Overall, the meaning of sanctification is the concept of separation from the secular and sinful and is set apart for a sacred purpose.
God does the separating and the consecration. He told ancient Israel: “You shall not profane My holy name, but I will be hallowed among the children of Israel. I am the LORD who sanctifies you” (Leviticus 22:32). Much like God sanctified the country of Israel, Jesus Christ does the same for people. “For both He who sanctifies and those who are being sanctified are all of one, for which reason He is not ashamed to call them brethren” (Hebrews 2:11). We can see what Paul wrote to the Thessalonians: “But we are bound to give thanks to God always for you, brethren beloved by the Lord, because God from the beginning chose you for salvation through sanctification by the Spirit and belief in the truth” (2 Thessalonians 2:13). Therefore, sanctification is needed in order to have salvation.