Jamieson, Fausset and Brown on Matthew 3:6

6. And were baptized of him in Jordan, confessing their sins--probably confessing aloud. This baptism was at once a public seal of their felt need of deliverance from sin, of their expectation of the coming Deliverer, and of their readiness to welcome Him when He appeared. The baptism itself startled, and was intended to startle, them. They were familiar enough with the baptism of proselytes from heathenism; but this baptism of Jews themselves was quite new and strange to them.