No matter how good-intentioned it sounds. While I feel that this principle is basic to our understanding of God's ways and, therefore, can stand on its own, I would like to make some modern-day applications to it. So we must seek His Word to see if that union would be God's intent. Your WORD is Truth. Unity can only come about when it is done in accordance with His express Word.
Idolaters are unbelievers.
Uzzah was killed because he touched the ark against God's express warning. We are not to touch the unclean thing - and idolatry is, most decidedly, an unclean thing. They were all well-intentioned and sincere. But what they did was not in accordance with God's Word and He was sorely displeased even to the point of fatal judgment. In 1 Samuel 15 we have a repetition of these principles.