John Calvin Commentary 2 Corinthians 9:6

"But this [I say,] He that soweth sparingly shall reap also sparingly; and he that soweth bountifully shall reap also bountifully." — 2 Corinthians 9:6 (ASV)

Now the case is this: He now commends alms-giving through a beautiful similitude, comparing it to sowing. For in sowing, the seed is cast forth by hand, is scattered upon the ground here and there, is harrowed, and eventually rots; and thus it seems as good as lost. The situation is similar with alms-giving. What goes from you to another place seems like a diminishing of what you have, but the harvest season will come, when the fruit will be gathered. For as the Lord considers everything spent on the poor as given to Himself, so He afterwards repays it with large interest (Proverbs 19:17).

Now, concerning Paul's similitude: He who sows sparingly will have a poor harvest, corresponding to the sowing; he who sows bountifully and with a full hand, will reap a correspondingly bountiful harvest. Let this doctrine be deeply rooted in our minds, so that whenever carnal reason holds us back from doing good through fear of loss, we may immediately defend ourselves with this shield — “But the Lord declares that we are sowing.” The harvest, however, should be explained as referring to the spiritual reward of eternal life, as well as to earthly blessings, which God bestows upon the generous.

For God rewards the generosity of believers, not only in heaven but also in this world. Thus, it is as if he had said, “The more generous you are to your neighbors, the more abundantly you will find God's blessing poured out upon you.” He again contrasts blessing here with sparing, as he had previously done with stinginess. Thus, it appears that it is understood to mean great and abundant generosity.