The Bible doesn’t specifically address participation in sporting events. While it is true that we are ordered to be separate from the world (John 17:14-15), it is also true that we have to live in the world (1 Corinthians 5:10). As Christians, we have to work (and play) daily with those who are unsaved. No matter what part (or position) we play in life, we are to be witnesses for Christ (1 Peter 3:15). We are to be examples of Christ for all to see (Matthew 5:14-16). We should take every opportunity to show others just what Christianity is all about. We should always keep our conduct above reproach.
Many professional sports heroes today are a far cry from being positive role models. Steroids, illegal drugs, and alcohol seem to be the norm. It is common to hear of players raping women, cheating on or even beating their wives, attacking fans, and even assaulting people. Yet they still keep playing and receiving millions of dollars annually. It would seem that a Christian playing sports would truly stand out in this crowd.
If a Christian athlete is really praying and seeking a way to share Christ with others, God will provide ample opportunities. It could be sharing Jesus or praying with teammates. It could be praising God in an interview. It could even be witnessing to fans. Basically, it all boils down to this: whatever you do, whatever sport you play; do it for the glory of God (1 Corinthians 10:31).
The only caution to be given to a believer who is playing sports—or is a fan of sports—is to not allow sports to become an idol (1 John 5:21). Never allow sports to become more important to you than your relationship with God. It is fine to be interested in sports, to be knowledgeable about sports, to be a fan of a certain team. The problem is that there are far too many people who can name all of the NFL teams, but cannot name the books of the New Testament. There are far too many people who will spend hours playing and/or watching sports each day, but do not spend more than a few minutes a day praying or reading the Bible. Clearly, this is not pleasing to God. Our priorities in life should be ordered in such a way that they honor and glorify Him.