What does Romans 11:35 mean?

Paul's hymn about how God is vastly unknowable began with a declaration about the depth of three things: His riches, wisdom, and knowledge. That poem continued with three questions about what we can know of God and what He needs from us. The first two of those questions were in the previous verse.

This is the third one, perhaps pulled from Job 41:11: Who has given a gift to God that God needs to pay back? In other words, who could ever give to God anything valuable enough that God would be in their debt? Paul is underlining the hard truth that God simply does not need us. He needs nothing from us, and does not require our existence. He will exist in completeness for eternity no matter what we do or don't do in response to Him. He owes us nothing.

That's not necessarily a comforting thought. This, however, is reassuring: the God who owes us nothing has given to us every good thing (James 1:17). Even better, the God who owes us nothing has given Himself to us in the form of Christ Jesus. In the context of Romans 11, He has shown us baffling mercy instead of the punishment we deserve for our sin, based only on our faith in Christ.