“Desert,” in Greek eremos, hence hermit. St. John the Baptist is praised by St. John Chrysostom, as a perfect model, and the prince of an Eremitical life. (Hom. i. in Mar. and hom. i. in J. Bap.) Several sectarists do not approve of what St. Chrysostom advances in favour of an ascetic life, and doing penance for past sins. (Bristow)

Do penance.[1] Beza would have it translated repent. We retain the ancient expression, consecrated in a manner by the use of the Church; especially since a true conversion comprehends not only a change of mind, and a new life, but also a sorrow for past offences, accompanied with self-denials, and some severities of a penitential life.

The kingdom of heaven, which many times signifies the present condition of Christ’s Church. (Witham)

In this and other places of holy writ, instead of “do penance,” Protestants give “repent ye;” but general use has rendered metanoia, by pœnitentia, or penance; and in this text, not any kind of penance, or grief for sins committed, but that which is joined with a desire of appeasing Him who has been offended by sin; and this also by some external signs and works. For as many as heard this metanoeite, obeyed the voice, received from him the baptism of penance, confessed their sins, and it was said to them: Bring forth fruit worthy of repentance, ver. 8. Therefore, all this was contained in the penance preached by the baptist. And here we must not omit, that while sectarists preach faith alone, both the baptist and Jesus Christ begin their ministry with practising and preaching penance. (Tirinus)

Pœnitentiam agite, metanoeite. Which word, according to the use of the Scriptures and the holy fathers, does not only signify repentance and amendment of life, but also punishing past sins by fasting, and such like penitential exercises. (Challoner)