I think it's pretty simple. You nailed it when you said he's religious and you're not. That's the problem. That's a fundamental difference that is going to continue to manifest problems. I have little doubt his faith is the primary motivator behind wanting to get married before living together. You can't relate to that, since you don't share his faith. So, in order to find some measure of mutual understanding (a prerequisite to agreement) you will have to consider some secular arguments for not living together.
I lived with my wife for five years before we got married. Like you, I didn't see the problem with it. Like you, I "knew" she was going to be the woman I married and figured there was no problem with jumping the gun in terms of living arrangements. Indeed, there was no problem early on. But, two years after getting married, problems have begun to manifest. You see, by "playing house" before actually committing, you fool yourself into thinking you know what life with the other person will really be like. But marriage changes things. You know why? Because marriage is a public, formal, and legal commitment. When your commitment does not exist in a public, formal, and legal form, it really doesn't exist at all.
There are two things going on in the back of your mind that keep you from being committed outside of marriage. The first is the thought that you have a way out if things head south. Indeed, no matter how "committed" you think you are, if you aren't married, no one is going to question your decision to dump your boyfriend. Commitment is more than a feeling; commitment is an accountability. Without marriage, you don't have that accountability. The second thought that lingers in the back of your mind before marriage is the romantic fairy-tale image of the wedding itself, driving off into the sunset. That romantic image of something to come blinds you to present realities. Once the wedding has come and gone, you now only have the realities to deal with, not some fantasy. The difference between people who live together before marriage and those that don't is, those that don't live together first KNOW things are going to change after the wedding, while those that do live together first fool themselves into thinking things won't change. After all, that's the whole idea of living together first; you always hear people say, "You need to know what it's like. You need to test drive." Well, there's no test driving marriage. There's no getting a sneak peak. You can't try on commitment. You're either committed, or you're not.
The fact is, you are not committed to this guy. You may feel committed. And that's great. But, until you are actually married, you are not obligated and therefore not truly committed. So, you have to ask yourself, am I ready to be obligated to this guy. If the answer is no, then your feeling of commitment (the basis of your justification for moving in with him) is a romantic illusion. If the answer is yes, and he feels the same way, then there's no reason you shouldn't want to marry him now.
Note, these aren't moral judgments and have nothing to do with religion. Like I said, I did the shacking up thing. So have all of my friends. The overwhelming majority of people do it, because it seems to make sense. But it really doesn't, not when you think about the paradox of trial commitment.
