I don't have to be a Calvinist to understand the clear teaching of Scripture that says when we are born again into the family of God he will never leave us, forsake us, cast us out OR lose us. Now that last part gets missed. He will LOSE nothing but will raise us up again on the last day. That tells me that when we genuinely place our trust in Christ we become a new creation, a child of the living God, a joint heir in Christ. We are indwelled with the Holy Spirit of promise and he is our earnest payment - like a down payment - on our eternal life.
Now can a Christian go through times of doubt? Yes, I admit I did. Can a Christian go through times of rebellion against God? Yep, did that too. But does that mean I was saved, then wasn't, then was, then wasn't, and now am again? Nope. That isn't the deal. We have been sealed until the day of redemption. Only the King breaks the seal, we cannot. A Christian going through those times I described never really loses his faith, he never totally stops believing, because he can't. The Holy Spirit does not leave us EVER. There is always that Spirit bearing witness with our own that we are His. There may be times of doubt, of sin, of rebellion, but our Heavenly Father never stops loving us, never leaves us, never deserts us, and always draws us back to Him. The key to all this: does the person truly, in his heart confess that Jesus Christ is Lord and Savior? Has he really placed his trust in him to save him? Has he admitted he is a sinner who needs a Savior? When this happens, and only God sees the heart, then he has promised everlasting life. There is nothing we do to deserve or earn his grace and mercy. You said earlier you trusted in God's justice and mostly his mercy. Well, let me assure you his justice was satisfied when Jesus died on the cross. When he said, "It is finished.", it was, The price for our freedom had been paid and his resurrection from the dead was the proof that God accepted it. He offers us his everlasting mercy, love and grace. It is ours through faith.
A big AMEN to that...