From the very beginning, the enemies of truth launched an effort to infiltrate and confuse the people of God by mangling the truth and by blending lies with Christian doctrine… That was the case in the Corinthian church, where false teachers brought with them a quasi-Christian syncretism of gospel truth, Jewish legalism, and pagan mysticism. They were eager to blend the people of God with the pagan worshipers, and the truth of Scripture with the lies of Satan.
Undiscerning believers who partner in a common spiritual cause with unbiblical forms of Christianity or other false religions open the door wide to satanic corruption. The appearance of unity, no matter how enticing, is not worth sacrificing the clarity of the gospel. Furthermore, embracing those heretical systems falsely reassures their followers that all is well between them and God, when actually they are headed for eternal damnation. Partnering in a spiritual enterprise with unbelievers helps Satan muddy the doctrinal waters, and it cripples our ability to preach the need for repentance. Scripture is clear about how we are to respond when the very foundations of the Christian faith are under attack: our duty is to contend, not compromise.
– John MacArthur, The Cost of Compromise