The correct answer is: A good example is the best sermon.
Indeed, it is quite easy to be preachy about anything but it is actions that count, not words, to judge a person’s moral character. People generally tend to be more impressed and to follow one’s example than to follow one’s words. It is a very Christian principle since as the parabole in Matthew 7:15-20 states (New King James Version (NKJV)
“You Will Know Them by Their Fruits
15“Beware of false prophets, who come to you in sheep’s clothing, but inwardly they are ravenous wolves. 16 You will know them by their fruits. Do men gather grapes from thorn bushes or figs from thistles? 17 Even so, every good tree bears good fruit, but a bad tree bears bad fruit. 18 A good tree cannot bear bad fruit, nor can a bad tree bear good fruit. 19 Every tree that does not bear good fruit is cut down and thrown into the fire. 20 Therefore by their fruits you will know them.”
During Franklin’s time people were very religious and they would have immediately identified the Christian reference in this maxim.