What reason does the incarnation give for why Christians shouldn't excuse themselves from giving?
Because Christ, though infinitely rich, willingly left His place of absolute control, became poor, and humbled Himself as a slave for our salvation, believers who claim they cannot give have no valid excuse. The same self-humbling, self-giving attitude He displayed is the very attitude Christians are expected to take in their giving.
Paul uses the incarnation as the pattern for Christian giving. He says Christ left His area of being so rich in order to become poor, was found in the likeness of men, and humbled Himself — all of His own volition, not by compulsion. He became obedient even unto death, the death of the cross. Paul then applies this directly to giving: believers have to take that same kind of attitude. If God in the person of Christ was willing to leave that area of absolute control of the universe and take on Himself the role of a slave for our salvation, then no believer can say, "I can't give anything, I can't give so much." That reasoning is the basis for Paul's advice in II Corinthians 8: since they had begun before with a readiness to will a year earlier, they should now complete the doing of it out of what they have, so that the readiness of the will is matched by the performance.
For ye know the grace of our Lord Jesus Christ, that, though he was rich, yet for your sakes he became poor, that ye through his poverty might be rich.
But made himself of no reputation, and took upon him the form of a servant, and was made in the likeness of men:
More Questions
Why does Paul use Christ's humility as a reason for Christian giving?
Paul uses Christ's self-humbling as the model for Christian giving, so believers must adopt the same kind of attitude.
Why does Paul bring up Christ's humility when talking about Christian giving?
Paul uses Christ's self-humbling in relation to Christian giving because believers have to take that same kind of attitude.