The amount of people I've seen or heard go astray because of the person they've married is unbelievable subhan'Allah.
Usually it's because of parental pressure which leads them to marry someone who they don't want to. Initially they do it for the pleasure of their parents but then they realise it wasn't the right thing to do. With the threat of a failed marriage, they are forced to compromise on matters of deen but before long, their emaan suffers and it becomes normal.
Other times, a sister feels she is getting too old and must marry the next person who comes along. So she compromises and marries a joker for the sake of it; he is lost and he leads her astray.
Or sometimes they marry someone who seemed practising but actually wasn't. After marriage it becomes evident what the person is really like. Once again the person compromises to make it work.
My advice is never marry unless you're sure you know what you're doing. Check aqeedah thoroughly and don't marry someone who is clueless, regardless of pressure or blackmail.
Keep calm and marry a salafi. Not necessarily those who label themselves salafi yet their character and knowledge goes as deep as the latest refutation. But rather marry someone who spends time reading the books of the scholars and someone who has righteous friends; someone who has adopted the methodology of the salaf.
This person will be best. Not perfect nor anywhere near it. But at least they'll be grounded and insha'Allah they'll fear Allah. May Allah preserve our marriages upon goodness