I don’t know, of course, what your uncle’s real motive is in giving so generously to his church; it may not be what you suggest. He simply might want to support his church’s social work, or he may have some other reason for what he does.
But I’m afraid some people do hope God will reward them for their generosity (or for some other good deed they’ve done), either by blessing them right now or by granting them eternal life. Lurking in the back of their minds is the idea that at the last judgment, God will weigh their good deeds against their bad deeds, and if they have more good deeds than bad deeds then God will let them into heaven.
This, however, is a serious misunderstanding of what the Bible actually teaches. The Bible says that God is absolutely holy and pure, and even one sin — just one — would be enough to keep us out of heaven. In other words, we all stand condemned before God, and no matter how good we are, we can never be good enough to win our way into heaven. The Bible says, “There is no one righteous, not even one” (Romans 3:10).
This is why we need Christ, because he did something we could never do: On the cross he took upon himself the judgment that we deserve. Now by faith and trust in him we can be saved. May your uncle come to realize this and commit his life to Christ — and may you, as well.