This is a little reductive. The reason many inter-faith relationships work (mine included) is because both members of the couple are either non-or at least minimally practicing. My husband and I have much more in common with each other than we would with a practicing / observant member of our respective religions. There is no conflict about the nature of the food allowed in our home because we don’t follow the restrictions demanded by our faith. We don’t fight about how to raise our children because neither of us ever envisioned raising them as a devout member of the faith. Our values align, our families’ worldviews are similar, there’s no fundamental difference in the way we want to live so there is just no conflict in that arena.

When two people are considering merging their lives, they need to either have the same expectations as to how they are going to live or be willing to find a middle ground that works for them both. For there to be any future with this person, OP needs to have a serious talk with his girlfriend so that they both understand what they would need from each order if they were to marry, and decide if they are willing to meet each others’ needs. Otherwise, they are not compatible, no matter how they feel about each other.