Simply said, yes, it is a VERY big deal to be in partnership with a person who does not hold the same beliefs as you. Scripture is clear that this should not be.
Any kingdom divided against itself will be ruined, and a house divided against itself will fall. If Satan is divided against himself, how can his kingdom stand? I say this because you claim that I drive out demons by Beelzebul. — Luke 11:17, 18
For this relationship argument, Jesus’ words are what weigh heaviest for me. A kingdom divided against itself will be ruined. If this is true, how can the kingdom you will develop with your partner be built up strong and not fall if you are divided against yourselves from the beginning? You’re setting yourself up for failure.