Jamieson, Fausset and Brown on Matthew 3:2

2. And saying, Repent ye--Though the word strictly denotes a change of mind, it has respect here (and wherever it is used in connection with salvation) primarily to that sense of sin which leads the sinner to flee from the wrath to come, to look for relief only from above, and eagerly to fall in with the provided remedy.

for the kingdom of heaven is at hand--This sublime phrase, used in none of the other Gospels, occurs in this peculiarly Jewish Gospel nearly thirty times; and being suggested by Daniel's grand vision of the Son of man coming in the clouds of heaven to the Ancient of days, to receive His investiture in a world-wide kingdom (Da 7:13, 14), it was fitted at once both to meet the national expectations and to turn them into the right channel. A kingdom for which repentance was the proper preparation behooved to be essentially spiritual. Deliverance from sin, the great blessing of Christ's kingdom (Mt 1:21), can be valued by those only to whom sin is a burden (Mt 9:12). John's great work, accordingly, was to awaken this feeling and hold out the hope of a speedy and precious remedy.