The truth of the matter is, most Protestant denominations are filled with just as many false teachings and traditions as the Roman Catholic Church.
This idea of OSAS is just one such example and, I believe, is rooted in focusing too much on one aspect of a believer’s life and not the other aspects. They’ve focused so much on determining who is or isn’t a Christian and how to tell, that they’ve lost the focus on how believers are to live out their faith in practice.
They focused on “Saved by Grace, through Faith”, which is good, but forgot that works come from that faith and are a hallmark of a healthy faith.
What’s that saying again, let me show you my faith by my works?
Jam 2:17-18 KJV - “[17] Even so faith, if it hath not works, is dead, being alone. [18] Yea, a man may say, Thou hast faith, and I have works: shew me thy faith without thy works, and I will shew thee my faith by my works.”
That’s pretty clear to me.
I thoroughly agree and have said similar things frequently over the years on FR.