If I am not a follower of God, am I an enemy of God? What constitutes an enemy of God?
Jesus stated, "He who is not with Me is against Me, and he who does not gather with Me scatters abroad" (Matthew 12:30). The stand of God is of such a nature that you cannot take a middle ground position.
James warns, "Adulterers and adulteresses! Do you not know that friendship with the world is enmity with God? Whoever therefore wants to be a friend of the world makes himself an enemy of God" (James 4:4). An adulterer is a person who is married to someone by then seeks a sexual relationship with someone to whom they are not married. They are people who want the stability of a godly way of living while at the same time live in an ungodly way. It can't be done. You cannot mix the two.
It is this incompatibility that led Paul to say, "Do not be unequally yoked together with unbelievers. For what fellowship has righteousness with lawlessness? And what communion has light with darkness? And what accord has Christ with Belial? Or what part has a believer with an unbeliever? And what agreement has the temple of God with idols? For you are the temple of the living God. As God has said: "I will dwell in them And walk among them. I will be their God, And they shall be My people." Therefore "Come out from among them And be separate, says the Lord. Do not touch what is unclean, And I will receive you."" (II Corinthians 6:14-17).
So if a Christian's best friend is an atheist or a Muslim, they are sinners as well for incorporating themselves with the "ungodly?"
While Christians might have friends or acquaintances who remain in the world, and they might even be married to a non-believer (I Corinthians 7:12), there is too much difference in the way the Christian approaches life and the non-believer approaches life for the relationship to be strong. For example, I know people who spew profanity, taking the name of my God and trashing it. While I might work with them and like them for many things, it hurts me to be around them for any length of time. I think very carefully before entering into a business relationship with a non-believer because their standard of right and wrong does not match my own. I don't want a business partner's lies to bring down my life.
It would be very hard for a true Muslim, who is certain that a belief in a Godhead is one of the worse perversions of religion in the world, to have a close relationship with a true Christian. That one difference alone will be a source of irritation between the two people.
And face it, your idea of fun is likely to be very different from my idea of fun because both of our views of the world are colored by our beliefs.
The warning is not that Christians must separate themselves from living in the world. "I do not pray that You should take them out of the world, but that You should keep them from the evil one. They are not of the world, just as I am not of the world" (John 17:15-16). Christians live in the world, but they seek to keep the world from influencing them. Nor can a Christian in his dealings give approval to those who do evil. "Who, knowing the righteous judgment of God, that those who practice such things are deserving of death, not only do the same but also approve of those who practice them" (Romans 1:32).