Hi John,
OSAS / Eternal Security, is one of the five points of Calvinism, and is taught within it by the improper exegesis of scripture.
The fact is it is intrinsically linked to the entire TULIP doctrine so debating this aspect inevitably leads to the other four points.
Heb 6:4-6 and 10:38-39 demonstrates OSAS is not correct, as does 2 Thess 2:3.
What is important to note is that supporters of OSAS will equivocate on the word apostasy, to connote LOSE. We don't LOSE our salvation, but we can walk away from it as many have done, including one of the biggest evangelists that originally partnered with Billy Graham in the early days of their ministry. After experiencing Heb 6:4-6, he(Charles Templeton) walked away in a state of apostasy. Jesus said we will inherit eternal life IF we endure. That is up to us. If we give up there is no security of salvation, just certainty of eternal punishment.