The Christian Duck Test?

>If it looks like a duck, swims like a duck, and quacks like a duck, then it probably is a duck. There is arguably little difference between the lifestyle of the Stoic and the lifestyle of the Christian. Both eschew effeminacy in pursuit of virtue. Both find it best for man to worry about things he can control and to not worry about the things he cannot (Epictetus wrote of this. Christians call this "trusting the plan"; there is also the Christian serenity prayer which expresses this). Both believe in the cardinal virtues of Prudence, Justice, Fortitude, and Temperance (Christians certainly ought to believe these, and I'd bet most Stoics do believe them as well). I would argue that their way of life is in essence, the same. The question is this: if a Christian and a Stoic both **live the same exact life**, and the **only** difference in their essence is that the Christian believes in the gospel, then why does the Christian go to heaven while the unbelieving Stoic does not? More generally, the question is about the "mechanics" of Christian salvation: when one comes to believe in Christ (esp. when converting from stoicism) what is it that *actually* changes in the person's essence/soul/existence that makes him fit for the Kingdom of God? In other words, what is it about the intellectual position of being Christian that impacts the *state* of someone's soul?