To sanctify is to set apart something because it is sacred. In a Christian sense, it is that at the moment of salvation, you are set apart for the kingdom of God. There are three major phases of sanctification: conversion, the Christian life and eternity.
Sanctification starts at conversion. When you accept Christ’s gift of salvation, at that very moment you are set apart. Your name is written into the Lamb’s book of life and it will never be removed from it. This is the sanctification from Hebrews 10.10, “By this will we have been sanctified through the offering of the body of Jesus Christ once for all.” The sanctification at conversion is a one time setting apart.
The second phase of sanctification takes place throughout the Christian life. This is a long term sanctification. It is the process of becoming more and more like Jesus. Jesus said, “Sanctify them in the truth; Your word is truth” (John 17.17). It is through the washing of the word of God which we become sanctified—more like Christ. It’s funny that we use this word in this case because sanctification is setting apart and we have already been set apart, but this sanctification does show more and more that we are part of the kingdom of God as we become more and more like Christ.
Sanctification is completed when we die or are raptured. In eternity, we will not struggle with being unlike Christ, we will be perfect examples of His righteousness and therefore our sanctification will have been made complete. This is clear in Philippians 3.20, where it says that God “will transform our lowly bodies so that they will be like his glorious body.” The lowly body with the sinful nature will be gone and we will be complete in our sanctification with a glorious body like Jesus Christ.
The three phases of sanctification are really one in the same. They all work towards the same goal. In the first step, we are set apart for eternity as children of God. In the second, we are in process of becoming like Christ and in the final step, we are made perfect, exhibiting the full splendor of Christ for all eternity.