Kosta: “Only someone who is not sanctified can become sanctified.”
FK: “That is only if the concept of sanctification is a one time event. The Bible says it isn’t. Believers become more and more sanctified as they mature in the faith.”
If “sanctification” is the western equivalent of theosis, then certainly it is a process, at least for the overwhelming majority of us. Or is sanctification not theosis?
You, know as in the Lord's prayer...ἁγιάζω, to make holy, hallowed. See 1 Cor 6:11. It's all in the past tense.
I think they are certainly comparable, however there is a great difference between Latins (and most likely the Orthodox) and mainstream Protestants as to the significance of sanctification. As I understand it, Latins see sanctification as a process leading towards salvation, i.e. it precedes salvation. In the most common uses of the terms we would say that sanctification takes place after salvation.
Where I think there is similarity is that one description of sanctification I would use is it is the life-long process of being conformed to the image of Christ. Sanctification represents spiritual maturity and growth in faith.
I don't know if this is a difference or not, but I would also say that sanctification is an act performed by God upon believers. That is, we cannot sanctify ourselves. I'm not certain who the literal "actor" is with theosis.