5 Therefore, because He was so man, that the God lay hid in Him, there was sent before Him a great man, by whose testimony He might be found to be more than man. And who is this? “He was a man.” And how could that man speak the truth concerning God? “He was sent by God.” What was he called? “Whose name was John.” Wherefore did he come? “He came for a witness, that he might bear witness concerning the light, that all might believe through him.” What sort of man was he who was to bear witness concerning the light? Something great was that John, vast merit, great grace, great loftiness! Admire, by all means, admire; but as it were a mountain. But a mountain is in darkness unless it be clothed with light. Therefore only admire John that you may hear what follows, “He was not that light;” lest if, when you think the mountain to be the light, you make shipwreck on the mountain, and find not consolation. But what ought you to admire? The mountain as a mountain. But lift yourself up to Him who illuminates the mountain, which for this end was elevated that it might be the first to receive the rays, and make them known to your eyes. Therefore, “he was not that light.”
Source: Tractates on the Gospel of John (New Advent)