Am I obligated to divorce my Muslim wife who has reverted to Christianity, knowing that she has a child from me and European law grants her custody?

If the wife apostatizes from Islam, she has no right to custody of the child, because Islam is one of the conditions for custody. The husband must make every effort to prevent her from having custody of the child, due to the danger this poses to the child's religion. It is worth noting that marriage to a woman of the People of the Book is permissible in principle, but it carries great risks, especially in this era, so a Muslim should not embark on it.