Maybe the title is a bit of a stretch. I personally don't have any friends who actively worship the devil. But I can say that some of my very best and most loyal friends are non-Christians.
I understand that many people who grew up in church have inadvertently spent most of their time around fellow believers. Thus, a majority, if not all, of their friends are believers.
There are many crucial reasons why we need to broaden our friend circles to people who share different beliefs. Mainly because that's what Jesus did.
In Matthew 9, Jesus is hanging out with "tax collectors & sinners." The Pharisees are troubled by the site so they ask why Jesus is hanging out with such sinful people.
Understand that tax collectors back then were perceived as greedy people who amassed piles of money by collecting taxes from citizens (does that sound familiar? I digress).
Anyhow, Jesus answers "Those who are well have no need of a physician. . . For I came not to call the righteous, but sinners." Matthew 9:12-13
Jesus hits a grand slam with that response. If we're commanded to spread the gospel and love of Christ, how are we expected to do that when we're only in the presence of fellow believers?
Some of the most meaningful relationships you can have are with people who have differing, and even opposing beliefs. They'll fortify your faith in ways that fellow believers can't.
Take an atheist friend for example. I love talking with atheists about my faith. Being on the receiving end of their beliefs is just as enjoyable. It challenges my beliefs which will ultimately reinforce them.
Fellow believers use Scripture to encourage us to stay strong in our faith. Non-believers use their lack of faith to encourage us to seek answers that will bring us closer to God.
Don't be afraid to have your faith tested. To avoid hearing about opposing beliefs will put you in a dangerous comfort zone. If your faith isn't being "beaten up" from time to time, it can't grow stronger.
A good illustration would be when a blacksmith makes a sword. The metal must first be heated and beaten with a hammer before it can be made into a sword and sharpened.
One hard-to-answer question about God won't turn you away from Him. In the same way, one hard-to-answer question about the origins of the universe won't immediately turn an atheist away from what they believe.
Build relationships with non-believers and believers. Let the Holy Spirit work through. Then pray for them all.
God didn't equip you with His word for you to steer clear of a spiritual battle. He equipped you with it as a weapon to fight those spiritual battles. Be open to everyone.