One could delve into all types of arguments depending on their ideology, and that is why simplification is the only true and fair solution.
Tithing represents 10 percent of one's income. A proportional tax system does not have to reside at 10 percent. It can be higher or lower depending on the needs of the government. Because it would be applied proportionally to the entire population, the government would be unlikely to raise taxes to a very high level, because it would affect everyone and not just a small group of voters.
This is a clear advantage of a proportional tax rate where everyone participates. Again, a logical analysis of a system where 50 percent of the people pay no income taxes but are allowed to vote to make the other people pay more taxes makes absolutely no sense. If one goes back and analyzes not only tithing but many of the principles advocated in the Bible, I think they can rediscover what true fairness is.
I also strongly encourage you to read more about Dr. Ben Carson, especially his latest New York Times best-selling book America the Beautiful.