I’m a Christian and at times I am in a state of mortal sin. Thankfully I’ve always come to my senses, repented and vowed to amend my life. Was I not a Christian during the times I was in mortal sin?
According to your catechism, being so hostile to God as to be in Hell is more than just being in a state of mortal sin. What it says is that the mortal sin must be unrepented, which is different from the situation which you describe.
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1861 Mortal sin is a radical possibility of human freedom, as is love itself. It results in the loss of charity and the privation of sanctifying grace, that is, of the state of grace. If it is not redeemed by repentance and God’s forgiveness, it causes exclusion from Christ’s kingdom and the eternal death of hell, for our freedom has the power to make choices for ever, with no turning back. However, although we can judge that an act is in itself a grave offense, we must entrust judgment of persons to the justice and mercy of God.”
I cannot see how someone who is “excluded from Christ’s kingdom” could be described as a Christian, particularly because it is Christ who is excluding them. It sounds rather a lot like this:
Mat 7:21 Not every one that saith unto me, Lord, Lord, shall enter into the kingdom of heaven; but he that doeth the will of my Father which is in heaven.
Mat 7:22 Many will say to me in that day, Lord, Lord, have we not prophesied in thy name? and in thy name have cast out devils? and in thy name done many wonderful works?
Mat 7:23 And then will I profess unto them, I never knew you: depart from me, ye that work iniquity.
Those people may call themselves Christians, but Christ apparently doesn’t.