How long the time of John’s ministry was before he was shut up by Herod in prison the Holy Scriptures do not certainly tell us; but it must be very short, for our Saviour’s time was little more than three years, and we hear of his imprisonment in the beginning of our Saviour’s public ministry. All that we have of John’s ministry is to be found either in this chapter, or in Matthew 3:1-17, or in Mark 1:1-45, or in the John 1:1-51; John 3:1-36. From them all it appeareth, that the sum of his doctrine was, the necessity of repentance, and faith in Christ, in order to the remission of sins. His pressing faith in Christ is most clearly declared by the evangelist John. Matthew, Mark, and Luke insist more upon his preaching the doctrine of repentance for the remission of sins, and baptism as an evidence of it. Which doctrine or repentance he pressed both from evangelical motives, The kingdom of heaven is at hand, and from legal motives, or arguments of terror, The axe is now laid unto the root of the trees: in this setting an example to all ministers of the gospel, showing them what should be the main subjects of their discourses, for we shall find that our Saviour preached the same doctrine, and in the same method. What is here said we before opened:
See Poole on "Matthew 3:2". See Poole on "Mark 1:4". John did not preach that baptism was repentance, or that remission of sins was infallibly annexed to it, but that the way to obtain remission of sins was by repentance, and that baptism was an external sign and symbol of it.

See Poole on "Matthew 3:7", and following verses to Matthew 3:10, where we met with all this with no alteration, save that Matthew saith that he spoke this to the Pharisees and Sadducees, seeing them come to his baptism: though he did especially intend them, yet he spake in the hearing of the multitude, amongst whom they were.

A good and faithful minister of Christ should be one able to bring out of his storehouse things new and old, to give every one their portion in their season, and so courageous and faithful as not to be afraid to do it, nor for any reason decline the doing of it. Such was John the Baptist. These were the Roman soldiers, kept by them to maintain their conquest of Judea. Some of these also come to hear John the Baptist preach: hearing him press repentance, and bringing forth fruits that might testify the truth of it, they ask what they should do. John saith to them,
Do violence to no man, &c. Experience hath taught all people, that soldiers (especially employed to keep garrisons amongst a conquered people) are often very insolent, and for their own gain prone to accuse innocent persons, and the jealousy of conquerors often allows them too easy an ear; as also how apt they are by oppression to mend their short commons, or to exact upon others that they may spend luxuriously. All these are acts or species of injustice, which the Baptist lets them know must be left, if they would bring forth fruits fit for repentance. He doth not blame the employment of a soldier, but only regulates their behaviour in that employment. Wars in just causes are undoubtedly lawful under the gospel, and consequently so is the employment of a soldier; we read of several good centurions or captains of hundreds. But the soldier stands highly concerned to look:
1. That the cause be good in which he draweth his sword.
2. That he behaveth himself in it lawfully, not using any needless violence, not accusing any wrongfully, not endeavouring to mend his pay by any, rapine, or unjustly taking away what is another’s, either to spend in luxury, or to uphold himself in his station.
From this instruction of John the Baptist, we may learn several things concerning the nature of repentance.
1. That where there is a true root of repentance, it will bring forth fruits worthy of it.
2. That acts of mercy and justice are true and proper fruits of a true repentance, without which there can be nothing of it in truth.
3. That true repentance is best discovered by our abhorrence of and declining such sinful courses as we have formerly been addicted to, and have daily temptations to from the circumstances of our lives, and those callings, and places, and courses of life wherein the providence of God had fixed us.
4. That these things, repentance and faith, are such proper effects of both, as discover the truth of those gracious habits in the soul, and without which there can be no true evidence of them.

It being known to many what the angel had told Zacharias concerning John thirty years since, and what had miraculously happened at his circumcision, as also what Zacharias his father had prophesied concerning him; and there having been many who had observed the holiness and severity of his life all along, until he came to man’s estate; and knowing that the time was fulfilled for the coming of the Messias, the sceptre being now departed from Judah, and Daniel’s weeks being accomplished; and hearing him preach with that life and power which attended his ministry, as also considering his doctrine (not new in itself, being consonant to the Divine law, and the doctrine of the prophets, but) new to them, who had used to hear of rites and ceremonies and the traditions of the elders, but little or nothing of repentance, or bringing forth fruits worthy of it; they began to reason and debate with, themselves, whether John the Baptist were not the Messiah promised, and in great suspense they were about it. But John quickly satisfied them as to that, not desirous to arrogate to himself his honour, whose, messenger only he was.

Here is amongst critics a little dispute, whether our blessed Lord at his baptism (after which he soon began his public ministry) was full thirty years of age; ωσει and αρχομενος in the Greek give occasion to the doubt. Those who judge that he was thirty complete, conceive that the age before which the priests and Levites did no service in the tabernacle of God. Numbers 4:3 commanded the number of them to be taken from thirty years old to fifty, and it was done accordingly, Luke 3:34,Luke 3:35, &c. David, in the latter end of his life, so numbered them, 1 Chronicles 23:3, when their number (of that age) was thirty-eight thousand; yet in that chapter, 1 Chronicles 23:24,1 Chronicles 23:27, we find them numbered from twenty years old and upward; but possibly that was for some more inferior service. In conformity to this, most think that both John the Baptist and Christ entered not upon their public ministry till they were of that age; but whether they were thirty years of age complete, or current, is a question, but so little a one, as deserves no great study to resolve: the two qualifying words, ωσει and αρχομενος, would incline one to think Christ was but thirty years of age current, which is advantaged by what others tell us, that the Jews ordinarily called a child two or three years old as soon as it did but enter upon its second or third year. Some think our Saviour was ten months above twenty-nine years of age when he was baptized, after which he was tempted of the devil forty days before he entered the public ministry; but these are little things.
Being (as was supposed) the son of Joseph. Joseph was not his natural father, though so supposed by the Jews, Joseph being indeed his legal father, being married to the virgin when our Saviour was born, Matthew 1:20.