Jerusalem - The people of Jerusalem.
All Judea - Many people from Judea. It does not mean that literally all the people went, but that great multitudes went. It was general. Jerusalem was in the part of the country called Judea. Judea was situated on the west side of the Jordan. See the notes at Matthew 2:22.
Region about Jordan - On the east and west side of the river. Near to Jordan.

Jordan - Many of the best MSS. and versions, with Mark 1:5, add ποταμω, the river Jordan; but the definitive article, with which the word is generally accompanied, both in the Hebrew and the Greek, is, sufficient; and our article the, which should ever be used in the translation, expresses the force of the other.

Then went out to him (g) Jerusalem, and all Judaea, and all the region round about Jordan,
(g) The people of Jerusalem.

Then went out to him Jerusalem,.... The uncommon appearance of this person, the oddness of his dress, the austerity of his life, together with the awfulness and importance of his doctrine, and the novelty of the ordinance of baptism he administered, and the Jews having had no prophet for some hundreds of years, and imagining he might be the Messiah, quickly drew large numbers of people to him. Some copies read "all Jerusalem": that is, the inhabitants of that city, a very large number of them; and "all Judea", a great number of people from all parts of that country. "All" is here put for "many". And
all the region round about Jordan; multitudes from thence, which seems to be the same country with that which is called "beyond Jordan", Matthew 4:25 and is distinguished from Judea as here. The Septuagint in 2-Chronicles 4:17 use the same phrase the Evangelist does here, and likewise in Genesis 13:10.

Then went out to him Jerusalem, and all Judea, and all the region round about Jordan--From the metropolitan center to the extremities of the Judean province the cry of this great preacher of repentance and herald of the approaching Messiah brought trooping penitents and eager expectants.

There went out to him Jerusalem and all Judea. These expressions must be taken, not as meaning every individual, but as showing the wonderful impression produced by his preaching. All Judea, and among the rest, the people of Jerusalem came.