If you look up 'penance', 'penitence', or 'repentance' in the dictionary you will find that all these words are about doing something, often something unpleasant, in sorrow, or regret, or expiation of something wrong one has done in the past. If you look these words up in a good dictionary it will soon become very clear to you that it is the Christian Church that has given this meaning to these words.
In comparison with the weird and wonderful penances some of the other Christian saints have devised for themselves, the wearing of clothes made of camel-hair and the eating of locusts and wild honey might strike us as being at the saner end of the spectrum, though still unpleasant enough. When we read in today's Gospel that John the Baptist combined these life-style choices with the preaching of baptism of repentance for the forgiveness of sins we might suppose that we have traced to its source the connection between repentance and the doing of unpleasant things.
But we would be mistaken.
John's purpose in being clothed with camel's hair, with a leather belt around his waist, and eating locusts and wild honey was not to show us how to 'do penance'. His purpose was rather to identify himself as a prophet in the tradition of Elijah the Tishbite -- 'a hairy man, with a leather belt around his waist' (2 Kings 1.8). Just as Elijah opposed the kings of Israel who did evil in the sight of the Lord, so John the Baptist opposed King Herod (Mark 6.18). His rough food and clothing are meant to be, as Jesus recognised, a prophetic rebuke for those who wear soft, luxurious clothes and live in palaces (Luke 7.25).
John did not expect people to imitate his choice of food and clothing. When the multitudes and tax collectors and soldiers asked him what they should do in response to his preaching his replies to them were extraordinarily mild: 'he who has two coats, let him share with him who has none; do not over-tax, rob no one by violence or false accusation, and be content with your wages' (Luke 3.10-14).
John's preaching of repentance, then, did not require the doing of unpleasant things in expiation of past sins. In fact, it was not concerned with what had been done in the past at all, but rather with what was to be done in the future. Repentance, for John, meant being resolved for the future to change the way we see ourselves. From that change in our whole cast of mind will follow changes in the way we behave.
If we think of ourselves as people who should be wearing soft, luxurious clothes and living in palaces then, obviously, we will be anxious about what we shall eat and drink and what we shall wear (Matt. 6.25), and we will busy ourselves with relieving these anxieties.
But if we think of ourselves as people who seek first the Father's kingdom and his righteousness, then we will not need to be anxious about these things. We will know that life is more than food, and the body more than clothing; and that the heavenly Father who feeds the birds of heaven and clothes in glory the lilies of the field will feed and clothe those who are his children.
This giving up of anxiety about tomorrow is the beginning of true repentance, for once we are able to relax in the hands of the loving Father who creates us we will no longer be compelled to horde our clothing from those who have none, or to rob by extortion, violence or false accusation. Instead, we will be 'zealous to be found without spot or blemish, and at peace', living in 'holiness and godliness as we wait for new heavens and a new earth in which righteousness dwells' (2 Pet. 3.11-14).