Is once a Christian always a Christian?

Answer 1

• often that is the case,

  though very rarely there are conversions to different religions.

• Many people lapse from the faith, either to agnosticism/Atheism or to other faiths.
• Although one cannot 'undo' a baptism, it can be denied. Denying one's Christianity is a rejection of Christ, and as such would be seen by Christians as a bar to entry to Heaven.
• 2 Timothy 2:13 If we believe not, yet he abideth faithful: he cannot deny himself.

  No amount of denying or rejection of Jesus Christ AFTER someone is saved will make that person unsaved.

• This assumes that being baptised - usually without consent - 'saves' a person. Surely "faith" is required. That can be rejected. One can (and many do) reject the works and words of Jesus, and in doing so, one is surely not a Christian.
• This question is not about being "saved" after death. This question is about "being" a Christian - a status in this life, and not about any possible afterlife.

Answer 2

If Christianity wasn't just one of hundreds of religions in the world that claim to have the "right" answer it could make a claim of that nature. However many people come and go in almost every faith glad that they have finally found the truth. For Moslems a Christian convert who reconverts to Christianity from Islam would be an apostate - indicating that it can't be done. For atheists promises made to a religion are ephemeral once they have realized there is no deity and as binding as promises made the tooth fairy.In short "Once a Christian" is a note in your biography.