If churches needed to pay taxes, they would need more income to pay these. This can result in churches pressuring people to donate or even making it a rule that you have to pay to get into the church.
They would pay taxes only based on their profits, i.e. the donation revenue in excess of their expenditure. As a result, if they did not collect a lot of money that year, they would also pay less tax. So there is no way taxes would require churches to make followers donate more. The only reason they would need to do that is so that they could have more capital to spend.