Here's a couple...
If anyone comes in the Church teaching a foreign gospel, let them be eternally condemned.
I am astonished that you are so quickly deserting him who called you in the grace of Christ and are turning to a different gospel not that there is another one, but there are some who trouble you and want to distort the gospel of Christ. But even if we or an angel from heaven should preach to you a gospel contrary to the one we preached to you, let him be accursed. As we have said before, so now I say again: If anyone is preaching to you a gospel contrary to the one you received, let him be accursed.
Gal 1:6-9
If one comes teaching another gospel, whoever received and aids them will be held accountable as if they were taking part in their evil.
Everyone who goes on ahead and does not abide in the teaching of Christ, does not have God. Whoever abides in the teaching has both the Father and the Son. If anyone comes to you and does not bring this teaching, do not receive him into your house or give him any greeting, for whoever greets him takes part in his wicked works.
2 John 1:9-11
Keeping the same theme, if some come in the Church, teaching doctrine contrary to what has been taught by the apostles, we are not to listen to them or aid them in any way, but rather avoid them.
I appeal to you, brothers, to watch out for those who cause divisions and create obstacles contrary to the doctrine that you have been taught; avoid them. For such persons do not serve our Lord Christ, but their own appetites, and by smooth talk and flattery they deceive the hearts of the naive. For your obedience is known to all, so that I rejoice over you, but I want you to be wise as to what is good and innocent as to what is evil.
Romans 16:17-19
Amos 3, a good example of God punishing His people because they set up alters to other gods, their form of 'interfaith dialogue'. He shows how the Jews came together willingly with other religions, by asking such questions as...
"Do two walk together, unless they have agreed to meet?" Implied "No".
"Does a lion roar in the forest, when he has no prey?" Implied "No".
"Does a young lion cry out from his den, if he has taken nothing?" Implied "No".
"Does a bird fall in a snare on the earth, when there is no trap for it?" Implied "No".
"Does a snare spring up from the ground, when it has taken nothing?" Implied "No".
Then He declares judgment in verses 13 through 15:
Hear, and testify against the house of Jacob, declares the Lord God, the God of hosts, that on the day I punish Israel for his transgressions, I will punish the altars of Bethel, and the horns of the altar shall be cut off and fall to the ground.
I will strike the winter house along with the summer house, and the houses of ivory shall perish, and the great houses shall come to an end, declares the Lord.
The one real God don't take kindly to His people giving heed to false gods. His people are told to reject their gods, and rather teach them concerning the one true God. Not only is it a traitorous thing to seek common ground between the real God and false gods, but it's also the most unloving act we can do. John the Baptist declared that "
Whoever believes in the Son has eternal life; whoever does not obey the Son shall not see life, but the wrath of God remains on him." (John 3:36).
Those who are not forgiven, the wrath of God will remain on them, and if we give them any comfort in letting them think their way to God is also viable, they will likely die in their sins.
There is no common ground to be had. Either it's faith in the finished work of Jesus, His death, burial, and ressurection, for the forgivness of our sins and reconciliation to God, or it's something else completely.