Obviously the differences determine the relationship. If there is a difference on core salvation beliefs then there is cause of concern. For example the deity of Christ, the sufficiency of His blood to atone for sin and stuff like that.
Things like end times and the rapture are not as important but a solid belief in salvation by grace through faith cannot be glossed over.
I can associate with just about anyone but I can have fellowship with those who are genuine Christians.
Scripture does say to have compassion on those less fortunate. Not everyone has the blessing of good bible discipleship.
For the cause of Christ
Roger