6 The witness of the light then is true, whether it be manifesting itself or other things; for without light you can not see light, and without light you can not see any other thing whatever that is not light. If light is capable of showing other things which are not lights, is it not capable of showing itself? Does not that discover itself, without which other things cannot be made manifest? A prophet spoke a truth; but whence had he it, unless he drew it from the fountain of truth?
John spoke a truth; but whence he spoke it, ask himself: “We all,” says he, “have received of His fullness.” Therefore our Lord Jesus Christ is worthy to bear witness to Himself. But in any case, my brethren, let us who are in the night of this world hear also prophecy with earnest attention: for now our Lord willed to come in humility to our weakness and the deep night-darkness of our hearts: He came as a man to be despised and to be honored, He came to be denied and to be confessed; to be despised and to be denied by the Jews, to be honored and confessed by us: to be judged and to judge; to be judged unjustly, to judge righteously.
Such then He came that He behooved to have a lamp to bear witness to Him. For what need was there that John should, as a lamp, bear witness to the day, if the day itself could be looked upon by our weakness? But we could not look upon it: He became weak for the weak; by infirmity He healed infirmity; by mortal flesh He took away the death of the flesh; of His own body He made a salve for our eyes. Since, therefore, the Lord has come, and since we are still in the night of the world, it behooves us to hear also prophecies.
Source: Tractates on the Gospel of John (New Advent)