Q. What is Apostasy and can a true believer commit it?
A. The Greek word for apostasy means to fall away or defect in the sense of a divorce, so apostates are those who abandon the true faith. On the day we become a believer the Holy Spirit is sealed within us as a deposit guaranteeing our inheritance (Ephesians 1:13-14). God accepts responsibility for us and puts His seal of ownership on us (2 Cor. 1:21-22) Together the Father and Son wrap their hands around us (John 10:27-30) to prevent anything or anyone from taking us from them.
This tells us true believers can not commit apostasy. Therefore those who abandon the faith only appeared to accept it, and never gave themselves wholeheartedly to the Lord. In other words, they were never born again. When something new comes along, they are drawn to it.
“They went out from us, but they did not really belong to us. For if they had belonged to us they would have remained with us; but their going out showed that none of them belonged to us” (1 John 2:19)