I'll back this up. Often when you say something an unbeliever doesn't like or goes against their beliefs, they get "offended" and actively avoid you or become resentful and harbor anger against you. Many of them have no concept of forgiveness since they haven't experienced the forgiveness and love God offers.
This fact shouldn't stop you from telling the truth, but lay off the snark. Unbelievers often have deep sadness that fills them up because they really don't have much hope. They are often sad and anxious and in pain, even if they don't appear that way on the surface. Thus, Christians who lord the hope they have and become proud of it (i.e. I know the truth and you don't, I'm going to heaven and you're not) can be brutal and thoughtless to others.
That's not a commentary on @Gloria_Oldenpinner's post, just a general comment about the subject.
Eh, be careful with this. Only God can save people. Our job is to proclaim the gospel so that people can hear it and accept Jesus into their hearts and lives.
Note that I'm not endorsing running around on a mad crusade to prove people wrong - if that's your goal, you're probably going to get yourself in trouble. But this is the word of God. If you're a Christian and you're taking the opportunity to proclaim the gospel, you're going to end up proving a lot of people wrong. In fact, a lot of times challenges like that are how we do it. Paul himself did a lot of arguing in favor of the truth:
This passage is talking about the apostle Paul. Paul was a sinful human being, granted. But it still seems that there is a place for arguing and arguments in the Christian faith.
It all comes down to speaking the truth in love, which is the heart of the issue. Love is looking out for another person's true better interest, not what they want. And while it is in all unbelievers' better interest to hear the gospel, it's also in their better interest to have it presented to them in a gentle way that understands just how bad their life is without the gospel.