The account of the great light appearing at the baptism of Jesus is preserved in the Western Tradition sources from the pre-Constantine period, prior to the composition of both the Alexandrian and Byzantine Text families of the New Testament.
Matthew 3:15-16 And Jesus answering said unto him, Suffer it to be so now: for thus it becometh us to fulfil all righteousness. Then he suffered him. And Jesus, when he was baptized, went up straightway out of the water: and, behold, a great light shone round about from the water, so that all who had come were afraid.
This detail is not a later addition but part of the original memories from the eyewitness accounts. It is preserved in the Western Tradition—specifically in the Old Latin manuscripts (a and g)—and is explicitly testified to by Justin Martyr in approximately AD 150.
This detail comports well with the conclusion of this account of the events surrounding Jesus’ baptism:
Mat 4:14-16 … so that it might be fulfilled which was spoken by Isaiah the prophet, saying: 15 “The land of Zebulun and the land of Naphtali, by the way of the sea, beyond the Jordan, Galilee of the Gentiles, 16 the people sitting in darkness have seen a great light, and to those who sat in the land and the shadow of death, light sprang up on them.”
This “great light shone round about from the water” at the baptism of Jesus may very well be what the apostle John relates here about the water and the blood, the beginning and the end of Christ’s ministry on earth. (A great light at His water baptism and a great darkness at the shredding of His blood at crucifixion):
1Jn 5:8-9 And there are three that bear witness in earth, the Spirit, and the water, and the blood: and these three agree in one. 9 If we receive the witness of men, the witness of God is greater: for this is the witness of God which he has testified of his Son.
This calling of Jesus at the waters of baptism bears some resemblance of the calling of Saul of Tarsus song the road to Damascus, who was then transformed into Paul the Apostle:
Act 9:3-6 And as he was going, it came about that he drew near to Damascus, and suddenly a light shone around him from heaven. 4 And falling on the ground, he heard a voice saying to him, “Saul, Saul, why are you persecuting Me?” 5 And he said, “Who are You, Lord?” And the Lord said, “I am Jesus, whom you are persecuting. 6 “But rise up and enter into the city, and it will be spoken to you what you must do.”
Both men, Jesus and Paul, would go on to a brief period of fasting, and trial by weakness, before beginning their powerful preaching ministries accompanied by confirming miracles.
Paul explained this preaching ministry with Barnabas to the Jews and Gentiles gathered at the synagogue of Antioch of Pisidia:
Act 13:47-49 For so the Lord has commanded us: ‘I have set you to be a light of the nations, that you should be for salvation to the end of the earth.’ ” 48 Now when the Gentiles heard this, they rejoiced and glorified the word of the Lord. And as many as were appointed to eternal life believed. 49 And the word of the Lord was being spread throughout all the region.
Peter also had an encounter with the supernatural light:
Act 12:7 And behold, an angel of the Lord stood there, and a light shined in the prison room; and striking the side of Peter, he roused him saying, “Arise quickly!” And his chains fell away from his hands.
I want you to know that each one of us who believe, who take seriously, Jesus Christ are affected by this great light.
Joh 12:44-46 Jesus cried out and said, “He who believes in Me, believes not in Me but in Him who sent Me. 45 And he who sees Me sees Him who sent Me. 46 I have come as a light into the world, so that whoever believes in Me may not abide in darkness.
Since God is your Father, what does that make you?
Jas 1:17 Every good gift and every perfect gift is from above, and comes down from the Father of lights
Eph 5:8 For you were once darkness, but now you are light in the Lord. Walk as children of light
1Pe 2:9 You are a chosen race, a royal priesthood, a holy nation, a people for God’s own possession, that you may proclaim the excellent virtues of Him who called you out of darkness into His marvelous light
Mat 5:14-16 You are the light of the world. A city that is set on a hill cannot be hidden. 15 Nor do they light a lamp and put it under a basket, but on a lampstand, and it shines on everything in the house. 16 Thus let your light shine before men, so that they may see your good works, and they may glorify your Father who is in heaven.