The death of Christ on the Cross didn’t confirm for you your sins were forgiven if you believe?
If we believe, we will believe every word. If we all appreciated Jesus greatness, we would all be too humbled than to assume knowledge of the hearts of others, or the self proclaimed entitlement of God's Salvation.
I don't pretend to be all knowledgable but when it comes to discussions about our Father in heaven, I have no doubt that humility and kindness are what real Christians will offer.
We believe Christ died for our sins. But we also know that all humans, following in human nature after Adam and Eve are sinful. Thus we do not believe in OSAS.