Can I make a custody agreement that says my child will be raised Christian?

No.  While this may seem strange to many parents,  it is contrary to the First Amendment, to make an agreement between parents controlling the religious upbringing of their minor children. California Courts have invalidated such agreements on public policy grounds because such agreements (i) are too vague to demonstrate a meeting of the minds or to provide an adequate basis for enforcement and (ii) excessively entangle the court in religious matters. 

Contact me at amanda@gordonfamilylaw.com for more information.