"Examine yourselves to see whether you are in the faith; test yourselves."
This admonishment from Paul gives us the opportunity to discuss the two types of sanctification that are at work in every believer.
The first work of sanctification happens at the moment of our conversion and salvation. It is Christ's work in us, His righteousness bestowed upon us the minute we admit our sin, repent and accept Christ's atonement on the Cross as payment in full for the sin debt and death penalty that was ours to pay. This instant sanctification immediately and permanently places us in the position of being in right standing before God, adopted into the familiy of God and establishes us forever as co-heirs with Christ of the Kingdom of God. It is a work of Christ that, once genuinely received, cannot be undone.
The second (yet simultaneous) work of sanctiication is called progressive sanctification and it is the ongoing process where we work in partnership with the Holy Spirit to be daily conformed into the likeness of Jesus, in word, thought and deed. Whereas the first work of sanctification is solely the work of Christ in us and nothing we could ever do can change our position in Christ, it is how we submit to the wisdom of God's word and the leading of the Holy Spirit to align ourselves with God's will (and our humble confession and repentance of sin when we choose to live by self will) that determines and effects the level of intimacy and clarity and relationship that we experience at any moment in communion with the Father. To be sure, our position in Christ is settled forever, yet we must daily strive to maintain our close and unhindered relationship with the Father through a life of love and obedience. This is not works unto salvation, but the fruit that is produced as a result of our working out our faith after salvation.
To use the example of family dynamics to illustrate the point, My son or daughter could never do anything to disqualify themselves from their position as my children. That is who they are by vitrtue of their birth into the family and it is their unchangeable position. No matter what they could ever do in life, they could never be un-soned or un-daughtered. That is a picture of positional sancification. However, the relationship that they enjoy with their parents can absolutely be damaged, strained and disconnected due to disobedience, disrespect and rebellion. Though still occupying their position in the family, they are forfeiting intimacy in that family relationship, and will continue to suffer it, until such time that they repent and re-align themselves with the the expressed will of their parents. Daily, they are learning how to live their life in such a way that they enjoy all of the intended blessings that right relationship is intended to bring. This is a picture of progressive sanctification.
It is the latter to which Paul appeals. His exortation to the church is for us to constantly check our hearts, check our thoughts, attitudes and actions and measure them against the Word of God and the heart of God to confirm that we are indeed walking in the faith that Christ purchased for us, being conformed into His likeness and experiencing the fullness and fruitfullness that is His intended blessing for us.