It is what a person believes about the person and work of Jesus that determines what their faith is in, and therefore, whether or not someone is in Christ.
What is atrocious
@Poly, is for someone to directly and deliberately sow seeds of doubt within the heart of another. Do you not know that we will reap what we sow?
Those who believe that the Bible teaches that it is God, and no part of themselves, who saves, even in His grace of giving them a new birth wherein their natural enmity towards God is dissolved, their stony heart removed and a new soft and pliable in His hands heart given; these are just as saved as those who (one could say arrogantly and with presumption) say they are saved because of what Jesus did but also because of what they did, when they, being much better people by nature than those who who do not chose to believe, chose to believe in Jesus. That sure Jesus did the work, made the sacrifice, but that it has no efficacy without the actions of man also.
All who are saved are saved by the grace of God.