John Gill Commentary Mark 1:5

"And there went out unto him all the country of Judaea, and all they of Jerusalem; And they were baptized of him in the river Jordan, confessing their sins." — Mark 1:5 (ASV)

And there went out unto him all the land of Judea
The people of the land, a great number of them:

and they of Jerusalem ;
the inhabitants of Jerusalem, hearing of this new preacher, the new doctrine that he taught, and the new ordinance that he administered:

and were all baptized of him in the river Jordan, confessing their
sins ;
that is, as many of them as were brought to a sight and sense of their sins, and made a confession of them, these he baptized, or immersed, in the river Jordan; for certain it is, that there were many of the Pharisees and Sadducees who came, whom he rejected; (See Gill on Matthew 3:5), (See Gill on Matthew 3:6), (See Gill on Matthew 3:7).