Repentance is both a change of thinking and a change in action. It is both abstract and concrete. Because of the internal part, which is both necessary and first, it may be tempting to treat repentance as an invisible thing. True repentance takes place in the heart, but is that all we can know?
Repentance is internal but it is not invisible, at least not indefinitely. Repentance on the inside—again, where it must begin—will work itself out. It will be external and so it will be visible. A fruit tree may be healthy without producing fruit, but only for a time. Likewise, the sap of repentance will produce a bucket worth of behavior.
John the Baptist used this type of terminology when he addressed a crowd that came to him to be baptized. He called them a “brood of vipers,” and he admonished them,
Bear fruits in keeping with repentance. And do not begin to say to yourselves, ‘We have Abraham as our father.’ For I tell you, God is able from these stones to raise up children for Abraham. (Luke 3:8 ESV)
There ought to be fruits, plural, and fruits “in keeping with” or “worthy of” (KJV) or consistent with a change of mind.
It’s not as if they had done nothing at all. They came to John, they said they wanted to be baptized by John. But he knew that more was necessary. When they asked, “Like what?” he told them to share their clothing and food with the needy (verse 11), that the tax collectors should be honest (verse 12) and that soldiers should not use their force for personal gain (verse 13).
The point is, we can’t lift up our desires to first place, or seek our own advantage, or use our opportunities to serve self. Selfishness is what we need to repent from, and it will be obvious to others.