I don't think that cussing/profanity determines a Christian. If we repent of our sins and ask the Lord into our lives...that is the main factor in determining whether you are a Christian or not. :) However, cussing/profanity DOES show a person's spiritual maturity, I think. I know cussing's really hard to get away from...but the Bible DOES tell us not to cuss.
James 3:10 - From the same mouth come blessing and cursing. My brothers, these things ought not to be so.
Ephesians 4:29 - Let no corrupting talk come out of your mouths, but only such as is good for building up, as fits the occasion, that it may give grace to those who hear.
Colossians 3:8 - But now you must put them all away: anger, wrath, malice, slander, and obscene talk from your mouth.
Matthew 15:11 - It is not what goes into the mouth that defiles a person, but what comes out of the mouth; this defiles a person.”
James 5:12 - But above all, my brothers, do not swear, either by heaven or by earth or by any other oath, but let your “yes” be yes and your “no” be no, so that you may not fall under condemnation.
After we allow Jesus into our hearts and claim to be Christian, we need to live up to Christ's standards, at every point possible. Christ didn't cuss, so we shouldn't either. And if we do, we should try to curb our tongues.
Romans 12:2 - Do not be conformed to this world, but be transformed by the renewal of your mind, that by testing you may discern what is the will of God, what is good and acceptable and perfect.
Hebrews 10:26 - For if we go on sinning deliberately after receiving the knowledge of the truth, there no longer remains a sacrifice for sins,
As Christians, our mindset needs to change. We need to be willing to change to fit Christ's standards, not worldly standards. Cussing is worldly.
Imagine this scenario, too: A Christian and a non-Christian come up to each other and start talking (male or female...doesn't make a bit of difference. Cussing is cussing). If the Christian is talking along, cussing like the non-Christian, and the Christian starts witnessing to the non-Christian about cussing, why would the non-Christian take his word to mean anything?
Non-Christian: Then why do you cuss all the time?
Christian: Oh, well...(No good excuse for that one!)
Non-Christian: ...>.> Yeah, whatever.
Can't witness to someone if they don't think you're living by it...right?