Baptism by Fire

Teaching English can be awkward if you are not constantly aware of the cultural, religious, ethnic and other differences that are present in most every class. So it is with great care that we explain idiomatic phrases, proverbs and other sayings that are often decidedly Judeo-Christian in nature. Take the expression "Baptism by Fire". For non-Christians the idea of a baptism can be a very foreign concept. So how do you explain this concept without prosetlyzing?
There are many approaches that one can use. We like to use the tried and tested method of KISS: keep it Simple Student.