While I don’t believe it’s a sin to marry an unbeliever, I do think it’s a bad idea in general. Acknowledging that yes, people can change either direction (me!), a Christian marriage works best when both parties are on the same page.
When my husband and I started dating, we hit it off pretty quickly, and I did start going to his church (I had not been going to my own in a few years - I was not really practicing faith). We studied with a group of young adults for a while, and I eventually chose to be baptized (immersed), which I’d previously not done. A month later, my then boyfriend proposed. He’d been waiting until I had made that faith choice. It was important to him to marry a Christian who believed as he did.
We committed ourselves to marriage. We don’t believe in divorce or remarriage after divorce (except for the case of adultery), so that has helped us weather the storms of marriage. In fact, even when I fell away and became an atheist, my husband’s commitment to the marriage was incredibly important. I am back in the church now, and we are united in how we teach our children. Let me tell you, it’s so much easier to raise faithful children when both parents are on the same page. We also have dealt with marital difficulties by going to the Bible. If we didn’t have that as a shared resource, some of those situations would have been more difficult to navigate.
Our faith is a major part of our daily lives. Not sharing that faith did immense damage to our marriage and our family life. When we were on the same page again, things in both realms improved greatly. We are not Sunday-only Christians (or even worse, Christmas and Easter-only Christians!). Our faith is tightly integrated into everything we do. Therefore, it’s vitally important that we believe in the same God and believe in the same methods of worship.
On the topic of this forum, my husband is undecided about old earth and evolution, and he chooses not to worry about it. He is fine with me teaching the kids mainstream science (our church teaches YEC). His opinion is basically that it doesn’t matter and is something he can’t know, so he won’t worry about it. I have to make a decision though, since we homeschool and I teach the kids science. I want to always pursue truth, and YEC is not truth. I’m thankful that my husband supports me in this endeavor.
At my church, there are a couple women whose husbands are not believers. My own husband’s father is not a believer. The heartache I’ve heard from all of these women about not sharing faith with their husband is enough for me to generally counsel young people to not marry a non- Christian. Again, I don’t think it’s a sin (if it were, Paul would not have told people to stay married to an unbeliever in 1 Corinthians), but I don’t think it’s a good idea either. That’s something I have drilled into my kids… find a spouse that shares your faith.