Guardian Angels are Nondenominational
Angels aren't just for Christians. Guardian angels work with people of all faiths: Jewish, Pagan, Hindu, Buddhist, Muslim, and, of course, Christians! Angels also work with people who are spiritual but do not identify with any one religion.
If everyone was assigned guardian angels before they were born, this makes perfect sense. It's also worth noting that angels do not have one favored spiritual tradition. The angels are mainly concerned with the Golden Rule: Do unto others as you would have them do unto you.
How about atheists? Do they have guardian angels? Yes. However, because we are powerful spiritual beings given free will by Spirit, angels normally respect our free-will choices to believe about this life, and navigate it, how we see fit. As long as someone's beliefs aren't hurting themselves or anyone else, angels respect these beliefs, and urge you to do the same.