My background: me strong athiest, her semi-religious Catholic. I had a high-school sweetheart, dated her for 5 years, married her, 6 years later she cheated repeatedly, now divorced. Not once in our 11 years was religion an issue.
Of course, since I'm divorced, that might disqualify my comments. But I swear that religion was never a problem. When she asked me to go to church, I went. Not because I believed in religion, but because I supported her and her beliefs.
You did hit the nail on the head though, "people change over time". For 9 years of our relationship I was adamant that we were the perfect couple, that our marriage would last forever, and that we weren't too young. The last two years changed all of that. She changed, she changed a lot. She changed so much that she no longer needed/wanted me in her life. So she basically refused to spend a minute of time with me to pursue her various boyfriends in the last 2 years of our marriage. I eventually caved and filed for divorce.
I urge you to wait for marriage. Not on religious reasons, but because the two of you haven't stopped changing yet. Give yourself a couple more years. Yes, I thought 5 years of dating was an eternity and couldn't wait for marriage. But I really wish I never got married. If by the time you are ~25 and you are still in a good relationship with her, then marry her. But let the big changes happen while you are still single and relationships are far easier to end.