Sanctification is a concept that is greatly misunderstood and causes a lot of disagreement between Christians.
One main reason for the problems surrounding this concept is that the word refers to 2 aspects of purification: God’s part and our part. Some people deny that we have any role in our sanctification, and others act like God has no role in this process.
So, for example, we read how God, in the Old Testament, sets apart some objects for His use. These are then sacred objects and are “sanctified” by God. To use one of these objects for ordinary use would be a wrong action. Likewise, in the New Testament we are told that we “are sanctified by the blood of Jesus”. When used this way the word refers to something that God has done. It is past tense; completed and finished. We play no role in this side of the equation. This is, of course, good news since it means that we are (already) purified and don’t need to “do” anything to approach God. Some people call this type of God initiated and God completed actions “passive purification” since, as far as the human is concerned there is no active work involved, so the person receives the benefit passively.
On the other hand there are many places in the Old and New Testaments where God asks humans to “sanctify themselves”. There are many things that the human must “do” to obey this command. In the New Testament context the command to sanctify ourselves means to set ourselves apart for His service. This then includes us using our will to obey Him. For example, we are told to “resist sin”, “resist the devil”, “draw near to God”, “walk in the Spirit”, “crucify the flesh”, “mortify our lower nature”, “put on the new man”, etc. Now these are examples of “active purification” where we must do something in order to be sanctified.
Furthermore, it turns out that since humans cannot keep the law by will power alone that even the active parts of our sanctification cannot be done without God doing some work too. The difference between legalism and obedience is that legalism is an attempt, by human resources alone, to obey God’s commands; whereas obedience is accomplished by turning to God, admitting weakness, and following Him into a new life. Without God’s action in our walk we would not be able to “fulfill the law”. We must have the indwelling spirit within us and we must follow this spirit. A good way to summarize this is to say that our progress in sanctification is not legal but relational.
Lastly, it must be appreciated that prayer plays an important role in sanctification. That is, a human needs to spend time with God and be transformed by knowing Him in order to progress in holiness and obedience.
We do not merely learn rules and apply our will, we sit before God and “return His loving gaze” in order to become more like Him. Or, we sit before God and listen to what He reveals to us. Often revelation can show us both our old man and our new man and allow us to see ourselves. This is a great aid in sanctification.
For more information on this topic see the essay series that begin with fire.
See also Stage One of Monastic Purification.