St. Thomas Aquinas on the testimony of St. John the Baptist

“If I were to bear witness to myself, my testimony would not be valid. But there is someone else who testifies on my behalf, and I know that the witness he bears on my behalf is true (John 5:31-32)

John was perfect in his nature because he was a lamp, i.e., enriched by grace and illumined by the light of the Word of God. Now a lamp differs from a light: for a light radiates light of itself, but a lamp does not give light of itself, but by participating in the light. Now the true light is Christ: “He was the true light, which enlightens every man coming into this world” (above 1:9). John, however, was not a light, as we read in the same place, but a lamp, because he was enlightened “in order to bear witness to the light” (above 1:8), by leading men to Christ. We read of this lamp: “I have prepared a lamp for my anointed” (Ps 131:17).

Further, he was blazing and impassioned in his affections, so he says, blazing. For some people are lamps only as to their office or rank, but they are snuffed out in their affections: for as a lamp cannot give light unless there is a fire blazing within it, so a spiritual lamp does not give any light unless it is first set ablaze and burns with the fire of love. Therefore, to be ablaze comes first, and the giving of light depends on it, because knowledge of the truth is given due to the blazing of love: “If any one loves me, he will keep my word, and my Father will love him, and we will come to him, and make our home with him” (below 14:23); and “I have called you friends, because everything I have heard from my Father I have made known to you” (below 15:15); “You who fear the Lord, love him, and your hearts will be enlightened” (Sir 2:20).

The two characteristics of fire are that it both blazes and shines. Its blazing signifies love for three reasons. First, because fire is the most active of all bodies; so too is the warmth of love (charity), so much so that nothing can withstand its force: “The love of Christ spurs us on” (2 Cor 5:14). Secondly, because just as fire, because it is very volatile, causes great unrest, so also this love of charity makes a person restless until he achieves his objective: “Its light is fire and flame” (Sg 8:6). Thirdly, just as fire is inclined to move upward, so too is charity; so much so that it joins us to God: “He who abides in love abides in God, and God in him” (1Jn 4:16).

Finally, John had an intellect that was burning brightly. First, it was bright within, because of his knowledge of the truth: “The Lord will fill your soul with brightness,” i.e., he will make it shine (Is 58:11). Secondly, it was bright without, because of his preaching: “You will shine in the world among them like stars, containing the word of life” (Phil 2:15). Thirdly, it was bright because it manifested good works: “Let your light so shine before men that they may see your good works” (Mt 5:16).