Christians are called upon by God to share the Christian gospel with others in order to help them become Christians.
One of the ways Christians try to evangelize (help non-Christians believe the gospel) is to help them see how wretched they really are so that they might see their need for Jesus.
Others try to help people see their philosophical need for a source or ground of absolute truth. This can be done using various methods.
Here's the great commission from Matthew 28:18-20:
Matthew 28:18-20 ESV And Jesus came and said to them [the eleven disciples], “All authority in heaven and on earth has been given to me. (19) Go therefore and make disciples of all nations, baptizing them in the name of the Father and of the Son and of the Holy Spirit, (20) teaching them to observe all that I have commanded you. And behold, I am with you always, to the end of the age.”
When I worked at a coffee shop I observed a man in line make a rude comment and a woman who did not know him said, “There are ladies present.” He said, “Where?” She then spent the rest of her time in line explaining her lady-ness to him. Then she sat at his table. It blew my mind. I later learned from conversation with a co-worker to whom I explained this event that this is a flirting device known as “the neg.”