Explain what "Their reward multiplies means. "You are suggesting that people who give money to churches are rewarded ; what is the reward? How does that reward multiply? Is that reward a promise from God? Can you show us in scripture this reward being promised in the NT?
First, You are incorrectly trying to restrict the topic only to giving money to churches. That's not all the giving that God values.
Luke 6:38
Give, and it will be given to you. A good measure, pressed down, shaken together and running over, will be poured into your lap. For with the measure you use, it will be measured to you.”
God values generosity. Those who give generously are rewarded in many ways. Here is Jesus praising the practice of sacrifice and offerings, showing us God's perspective. Specifically, He says nothing about doing away with the practice.
Matthew 23:23
23 “Woe to you, teachers of the law and Pharisees, you hypocrites! You give a tenth of your spices—mint, dill and cumin. But you have neglected the more important matters of the law—justice, mercy and faithfulness. You should have practiced the latter, without neglecting the former.
Jesus specifically praising a person who gives to the temple treasury:
Mark 12:41-44
41 Jesus sat down opposite the place where the offerings were put and watched the crowd putting their money into the temple treasury. Many rich people threw in large amounts. 42 But a poor widow came and put in two very small copper coins, worth only a few cents.
43 Calling his disciples to him, Jesus said, “Truly I tell you, this poor widow has put more into the treasury than all the others. 44 They all gave out of their wealth; but she, out of her poverty, put in everything—all she had to live on.”
Now, if you're narrowly harping on whether Christians today are bound by God's law to rigidly give 10% of their increase to their local church, there's not too much to argue about. And, if your intention is to point out and condemn those pastors who live lavish lifestyles while their congregants suffer, again, not much to argue about. If, however, as I suspect, your true intention is simply to attack Christianity and you think you are somehow going to make the case that Christians joyfully giving to their Churches is not Scriptural, then you have already failed.