You know what upsets me? When people claim harder music is incompatible with the Christian faith. Not only are there some pretty spectacular, very Christian metal outfits out there (The Devil Wears Prada, Underoath, As I Lay Dying) but the same people who judge hard rock lovers for their "especially secular" tastes often also claim to like such bands/artists as Jack Johnson, Jason Mraz, One Republic, Michael Buble, et cetera, who address all the same worldly themes in their lyrics. Being mellow doesn't make music automatically more Christian. Listening to a secular hard rock band is the exact moral equivalent of listening to a secular acoustic singer/songwriter, and neither of these is inherently evil.
This brings me to my next point. From what I've gleaned about the world and about God (not speaking from a place of authority, just from my own experience), listening to secular music is not a one way ticket to hell, nor is it a necessarily bad practice for Christians to take part in. Most of the music I listen to happens to be either made by Christian bands or bands comprised mostly of Christians simply because I can relate to the lyrical content of the songs. When Run Kid Run sings, "I'm singing for freedom, I know I'm not the only one praying to the One who can bring me this freedom. I'm ready for change," I can connect to the message of the song on a deeply personal level. I listen mostly to Christian music because it is what I enjoy, not because I believe God demands I do. I also love a lot of secular music. Some of my favorite bands are secular (Breaking Benjamin, Incubus, Journey, Heart), and most of my other favorite bands are not classified as "Christian bands" (Anberlin, Paramore) but have members who are Christians. I generally steer clear of music that defiles or insults my Lord, but not all secular music is this way, and even then I do not necessarily condemn those who listen to it, but I cannot relate or connect to the music, so I choose not to listen.
I admire with all my heart those who devote their whole musical lives to worship music alone. Worship music taps into the original purpose of music itself (glorifying God) and it is such a valuable tool in my own faith and in the faiths of many others. To say total abstinence from non-Christian music is the only Godly path, however, I feel sort of misses the point. It reminds me of the pharisees, standing on the street corner, flaunting the merit of their own behavior and demanding others do the same. We should all pray to God about our entertainment choices, so He might guide us in this aspect of our lives along with all others, and if we feel clearance to participate in certain entertainment choices not in flagrant violation of the scriptures, then maybe it falls under the category of Christian freedoms (1 Corinthians 6:12).
Music is, after all, just another art form like theatre, cinema, paintings and the like. Listening to secular music is no different from watching secular movies or television shows (pretty much everything, the Office, the Lord of the Rings). All these things should be approached with knowledge and caution, not condemnation. Speaking of condemnation, maybe the world would see Christians as the loving people we are called to be if we loved first and judged last, tending to the planks in our own eyes instead of judging the specks in the eyes of others (Matthew 7:3). We should not be so sure we're better than others, we should "do nothing out of selfish ambition or vain conceit, but in humility consider others better than ourselves" (Philippians 2:3).
If God were to convict me of my behavior and I felt God would not want me listening to such music, I would get rid of it, no questions asked. Being intentionally disobedient to God's calling is a dangerous practice. People have been swallowed by whales for less. I think a good rule of thumb for Christians is if there is anything in our lives we would not get rid of if God asked us to, we should probably get rid of it right away. This does not only apply to music either. I believe this applies to many aspects of the Christian walk. If we prioritize the kingdom of God first, and God knows our hearts are in the right place, then dancing along to a pop tune on the radio may not spell damnation after all.
Though it may seem like a surprise to some, being tuned into the culture enough to be familiar with secular music may be an effective ministry tool. God uses our tastes and personalities to reach all sorts of people. If we conform to some sort of legalistic pattern, we would be unable to relate to some people we were meant to relate to. I don't mean we should sin so we can relate to sinners, but it is clearly not the same thing. I mean God made us with unique passions to serve different purposes, and together these passions combine to create one functioning body with Christ at the head.
And a final thought: If you think listening to Christian music and Christian music alone makes you look like a good Christian, then perhaps you are too concerned with looking like a good Christian. Christianity is not a show. It is a day to day relationship with the living God.
If you have any encouragement from being united with Christ, if any comfort from his love, if any fellowship with the Spirit, if any tenderness and compassion, then make my joy complete by being like-minded, having the same love, being one in spirit and purpose. Do nothing out of selfish ambition or vain conceit, but in humility consider others better than yourselves. Each of you should look not only to your own interests, but also to the interests of others.