"Be ye not unequally yoked together with unbelievers: for what fellowship hath righteousness with unrighteousness? and what communion hath light with darkness?"
The following excerpt is from a sermon on 2 Corinthians 6:14 delivered by Dr John MacArthur (4 part series available from http://www.gty.org/)
"What we're talking about here is any linking together with an unbeliever in any religious or spiritual enterprise. That's what we're talking about. We're not talking about mutual funds, you can rest easy. We're not talking about you should quit your job cause you work with non-believers. We're not talking about Christian pulling out of the school because he doesn't have a Christian teacher. We're not talking about leaving your neighborhood. We're not talking about any of that. We're talking about a spiritual enterprise, worship, ministry, evangelism.
Religious cooperation between the kingdom of darkness and the kingdom of light is ridiculous. Why would we want to give Satan access? You say, "Is this...is this a problem?" Is this a problem? This is Satan's number one ploy. I remember when I was a student in college, 1955, I was first confronted with the fact that huge massive evangelistic endeavors were being held in America and the committees were made up of Christians and non-Christians, people who believed the Bible and people who denied the Bible and were theological liberals. And I wasn't particularly profound, believe me, at that age. I was still in high school actually in '55, I didn't graduate till '57. But it was in those years and I was asking, "How can they do that? I don't understand how you can bring unbelievers and believers together in a common spiritual enterprise." It doesn't make any sense. I mean, why would you invite Satan in?
We still have that today. Satan still endeavors to encroach. Recently we had the Promise Keepers event in Los Angeles. And right around the time of the Promise Keepers I picked up the Los Angeles Times and found that the Cardinal--the Catholic Cardinal, had affirmed everything about the Promise Keepers and encouraged all the parish priests to take all their men. That was followed in an article, I think a day later, by the local Mormon bishop who said that he was encouraging all the Mormons to go. What does that say about Promise Keepers? Nothing. What it says about Satan is everything. That's always been his approach. He doesn't want to fight it, he wants to...what?...he wants to join it. When he fights the church it explodes and the blood of the martyrs becomes the seat of the church. When he joins the church it dies. And he always wants to get involved in it. And witless, reckless, undiscerning believers think that's an evangelistic strategy and embrace it."