Why should believers avoid doing things that make outsiders think they are strange?
Believers should avoid conduct that makes outsiders view the church as strange because God is a God of order, and His people should reflect order rather than contentiousness. The instruction is to have no such custom, nor do the churches of God, so believers should not do anything that makes the outside world think they are a bunch of kooks.
Les explains that Paul is tempering the attitude of believers concerning the haircuts of prostitutes compared to the Roman citizenry. In verse 16, Paul says if any man seem to be contentious, we have no such custom, neither the churches of God. Les applies this by saying, just don't do anything that makes the outside world think the church is a bunch of kooks. He grounds this in the fact that God is a God of order, seen in Israel's orderly camp, prescribed temple worship, and even the resurrections, where every man is in his own order. So the concern is maintaining an orderly, non-contentious testimony before outsiders.
But if any man seem to be contentious, we have no such custom, neither the churches of God.
More Questions
Why should Christians be careful how the world sees their church services?
Believers must be careful not to let the outside world view their church service as a bunch of nuts or people out in left field, because the world is watching them.