John Calvin Commentary Romans 15:28

"When therefore I have accomplished this, and have sealed to them this fruit, I will go on by you unto Spain." — Romans 15:28 (ASV)

And sealed to them this fruit, etc. I do not disapprove of what some think: that there is an allusion here to an ancient practice, where people sealed up what they intended to keep safe. Thus Paul commends his own faithfulness and integrity, as if he had said that he was an honest keeper of the money deposited in his hands, just as if he carried it sealed up.

The word fruit seems to refer to the produce, which he had previously said returned to the Jews from the spread of the gospel, in a way similar to how the land, by producing fruit, supports its cultivator.