How can one convince a person that Christians will enter Hellfire, even though they were born Christian and it is difficult for them to change their religion, especially the young ones whose parents prevent them?

Every human being is born with the فطرة (innate disposition) of Islam, which is soundness from false beliefs and acceptance of true doctrines. If one remains upon his فطرة, he will be affirming God and loving Him. As for one who reaches the age of accountability and persists in disbelief and dies upon it, he deserves the Fire for what he has earned, because God has given him intellect, hearing, and sight, and has guided him to the two paths so that he may choose his way.