First of all telling someone else to tithe is in poor judgment. Tithing is a form of personal stewardship between the individual and God. It was not invented by man or churches BUT is used/mis-used by both regularly.
The subject is addressed in the Bible. Simply stated, tithing was ordained by God as a method of supporting the gospel and as a faith excercise. Those who have great faith in God give generously and are generously rewarded. Those of little faith who give but little are rewarded little. Those who give nothing "rob God" according to scripture.
That being said, no one should judge anothers stewardship.