Here’s a second excerpt from my book, The Essentials of the Christian Religion, available here:
The second term, sanctification, refers to the process of actually making us righteous. As we said in the last chapter, we are saved from our sins, not merely from the consequences of our sins. Justification saves us from the eternal consequences of our sins, but it does not save us from the sins themselves.
After we are converted, however, we are adopted as sons by God and we are given the power to really be sons, rather than the enemies that we were before. This power comes to us by the Holy Spirit, who works internally in us to teach us and empower us to live more and more like Christians. This is sanctification. Sanctification means the process of setting something apart. The term was used in the Old Testament to describe the ‘setting apart’ of all the firstborn to God. When any sheep or goat or other animal that people had gave birth to its first offspring, that offspring was given to God as thanks for all God had done for His people. The way it was given to God is that it was sacrificed in the temple. Even firstborn children were said to belong to God, though God did not demand that they be killed. Rather, a substitute offering was to be given. A family was required to offer a lamb, or if the family was poor they could offer a pair of pigeons or turtledoves instead. This symbolized that all that people had was ‘set apart’ to God, that is, taken out of the regular, sinful world and dedicated to God.
Many other symbols in the Old Testament point to this same need: the Jews were forbidden to eat many foods and wear certain clothes that were said to be unclean. If they came into contact with things like dead bodies or unclean foods, then they became unclean too. If they were unclean, then they would not be able to enter the temple, and so they had to be cleansed with a ritual that involved the sprinkling of water.
All of this was just symbolic. God doesn’t care about pork or mixed fabrics. But He cares very much that we set our lives apart for His service. What was taught symbolically in the Old Testament is taught directly in the New, and while Christians are free from the Old Testament purification laws, the deeper reality that those laws pointed to—the need to be separate from the world—is every bit as important as it ever was. And so we Christians need to struggle and fight our whole lives to put away our sinful practices and renounce the ways of this world. Sanctification actually begins at the same moment that conversion occurs, since conversion itself is the beginning of putting away sinful ways and renouncing the world.
The Essentials of the Christian Religion is a survey of basic Christian doctrines, including an exposition of the Apostles’ Creed. It is written to be accessible to those uneducated in Christianity, and also refreshing and reminding for those more well-versed.