Matthew 3:6 - Wesley's Explanatory Notes

Confessing their sins — Of their own accord; freely and openly. Such prodigious numbers could hardly be baptized by immerging their whole bodies under water: nor can we think they were provided with change of raiment for it, which was scarcely practicable for such vast multitudes. And yet they could not be immerged naked with modesty, nor in their wearing apparel with safety. It seems, therefore, that they stood in ranks on the edge of the river, and that John, passing along before them, cast water on their heads or faces, by which means he might baptize many thousands in a day. And this way most naturally signified Christ's baptizing them with the Holy Ghost and with fire, which John spoke of, as prefigured by his baptizing with water, and which was eminently fulfilled, when the Holy Ghost sat upon the disciples in the appearance of tongues, or flames of fire.