"But this I say, He which soweth sparingly shall reap also sparingly; and he which soweth bountifully, shall reap also bountifully." (2 Corinthians 9:6)
As John Calvin pointed out long ago in expounding this key passage, "We are not giving, but sowing" when we contribute of our financial means to the work of the Lord, for it miraculously is considered by the Lord of the harvest as seed sown in the soil of the hearts of men.
And it is a rule of the harvest that, other things being equal, the more seed planted, the more harvested. He who is deficient with his seed must necessarily anticipate a meager crop.
Of course, a bountiful harvest presupposes not only an abundance of seed, but also good soil, properly prepared, watered, and cultivated. It is no good simply to give money to anyone or any cause, any more than it is good simply to throw a seed on a rocky slope or city street or weed-infested yard. One is responsible to give where God's Word is honored--not just to give, but to give responsibly.
Furthermore, even though an abundant harvest is promised, the motive in giving is also vital. The harvest is souls--not gold! "God loveth a cheerful giver"--not a conditional giver (v. 7). "He that giveth, let him do it with simplicity" (Romans 12:8). Often God does bring financial blessing to a Christian who has proved faithful in the grace of giving, but this is so he can give still more and thus lay up still more treasure in heaven. "For unto whomsoever much is given, of him shall be much required" (Luke 12:48). "Therefore," as Paul said, ". . . see that ye abound in this grace also" (2 Corinthians 8:7).
And as we give, we must never forget that Christ has given more: "For ye know the grace of our Lord Jesus Christ, that, though he was rich, yet for your sakes he became poor" (2 Corinthians 8:9). HMM