This is not the same thing. All those who call themselves Christian are not necessarily Christian. If you are a Scotsmen, there is nothing you can do about it, it’s in your blood. You are what you are.

““Not everyone who says to me, ‘Lord, Lord,’ will enter the kingdom of heaven, but only he who does the will of my Father who is in heaven. 22Many will say to me on that day, ‘Lord, Lord, did we not prophesy in your name, and in your name drive out demons and perform many miracles?’ 23Then I will tell them plainly, ‘I never knew you. Away from me, you evildoers!’”

If someone called himself a Christian but never really knew Jesus and Jesus never knew him, then his Christianity wasn’t authentic and he went through the motions without ever really meeting God. Therefore was not a real Christian.

If he was a real Christian then he would know there is a God and therefore could not be atheist, but a instead someone who turns from God.