17 Again, how weighty are the things which this evangelist reports Jesus to have spoken, when He came back to the temple from Mount Olivet, and after the forgiveness which He extended to the adulteress, who had been brought before Him by His tempters, as one deserving to be stoned: on which occasion He wrote with His finger upon the ground, as if He would indicate that people of the character of these men would be written on earth, and not in heaven, as He also admonished His disciples to rejoice that their names were written in heaven! Or, it may be that He meant to convey the idea that it was by humbling Himself (which He expressed by bending down His head) that He wrought signs upon the earth; or, that the time was now come when His law should be written, not, as formerly, on the sterile stone, but on a soil which would yield fruit.
Accordingly, after these incidents, He affirmed Himself to be the light of the world, and declared that he who followed Him would not walk in darkness, but would have the light of life. He said, also, that He was “the beginning which also discoursed to them.” By which designation He clearly distinguished Himself from the light which He made, and presented Himself as the Light by which all things have been made. Consequently, when He said that He was the light of the world, we are not to take the words to bear simply the sense intended when He addressed the disciples in similar terms, saying, “You are the light of the world.”
For they are compared only to the kindled light, which is not to be put beneath a bushel, but to be set upon a candlestick; as He also says of John the Baptist, that “he was a burning and shining light.” But He is Himself the beginning, of whom it is likewise declared, that “of His fullness have all we received.” On the occasion presently under review, He asserted further that He, the Son, is the Truth, which will make us free, and without which no man will be free.
Source: The Harmony of the Gospels (New Advent)