Adam Clarke on John 1:7

That all men through him might believe - He testified that Jesus was the true light - the true teacher of the way to the kingdom of glory, and the lamb or sacrifice of God, which was to bear away the sin of the world, Joh 1:29, and invited men to believe in him for the remission of their sins, that they might receive the baptism of the Holy Ghost, Joh 1:32-34. This was bearing the most direct witness to the light which was now shining in the dark wilderness of Judea; and, from thence, shortly to be diffused over the whole world.