I'm curious - is there a tradition from the early church (from Christ to 700 AD) about what John the Baptist said when he was baptizing people? How did he get that information?
The early Church record from Christ till the end of the 1st century A.D. does, indeed, provide information that opens up the subject of what John the Baptist said when baptising people.
It is essential, however, to first understand what his ministry was for - the point and purpose of him calling Jewish people to be baptised. That will prevent us looking in the wrong direction for what he said at the time of baptism. Of equal importance is to note that the very first Christian formula for baptising people was given by Christ himself, after he had been resurrected from the dead. It is recorded by the apostle Matthew in ch.28 vs.19:
"Go ye therefore, and teach all nations, baptizing them in the name of the Father, and of the Son, and of the Holy Ghost" (A.V.)
Of a certainty, John the Baptist never used that formula, for Christ had to have died and been resurrected first before that formula was issued by Christ himself, for his disciples to use, but (a good while before then) John the Baptist had been martyred. So, we can safely rule that formula out.
Was there an earlier formula used in Jewish circles for proselytes to the Jewish faith, who customarily were baptised after their conversion? If there was, it could never have been used by John because he was not gaining converts to the Jewish faith from amongst the Gentiles. His ministry was purely to Israelites. The purpose of his ministry was to prepare Jewish believers to receive the coming Messiah. This is where the three gospel accounts that speak of John the Baptist need to be combined to get the full picture.
Mark starts his account by saying that John's prophetic ministry was "the beginning of the gospel of Jesus Christ, the Son of God. As it is written in the prophets, Behold, I send my messenger before thy face, which shall prepare thy way before thee." (1:1-2) The key verses explaining the significance of John's baptism are:
"The voice of one crying in the wilderness, Prepare ye the way of the Lord: make his paths straight. John did baptize in the wilderness, and preach the baptism of repentance for the remission of sins. And there went out to him all the land of Judea, and they of Jerusalem, and were all baptized of him in the river of Jordan, confessing their sins." (1:3-5) Emphases mine.
The next verses tell us something of what John preached to those thousands of Jews - that they were to look for one coming, mightier than he. He added about this foretold one, "I have, indeed, baptized you with water: but he shall baptize you with the Holy Ghost." Still no baptismal formula for John's baptism, then.
Matthew's account is very similar, but with this interesting addition about what else John preached, specifically to the many Pharisees and Sadducees who also came to be baptised:
"O generation of vipers, who hath warned you to flee from the wrath to come? Bring forth therefore fruit meet for repentance. And think not to say within yourselves, We have Abraham to our father: for I say unto you, that God is able of these stones to raise up children unto Abraham. And now also the axe is laid unto the root of the trees: therefor every tree which bringeth not forth good fruit is hewn down, and cast down into the fire. I indeed baptize you with water unto repentance: but he that cometh after me is mightier than I, whose shoes I am not worthy to bear. He shall baptize you with the Holy Ghost and with fire..." (3:7-12)
Luke's account is very similar to Matthew's. It does have this addition, however. After that cutting address, the people asked him, "What shall we do then?" The reply first told them to share with the needy; publicans were told not to exact more tax than was legal; soldiers were told to do violence to no man and to be content with their wages. Not a word about any baptismal formula.
However, we now have enough information to come to this conclusion. It was John's preaching that was vitally important, and it was who he pointed to - Messiah - that the people had to look to. To be baptized of John, they had to confess their sins. This was part of his preaching "the baptism of repentance for the remission of sins". This was essential preparation for them to receive the Messiah who shortly appeared. This makes John the Baptist's preaching and baptism unique. That is why there is no mention of a formula that he used being written down anywhere. Even if he had used a formula, it would apply only to those people at that time, in order to prepare them to turn in faith to Jesus Christ. And not all of them would have saving faith, as pointed out here:
"Just so there went out to John all the land of Judea, and they of Jerusalem, and were all baptized of him in the river of Jordan, confessing their sins. Among this mixed multitude, God's hidden remnant lay concealed..." Mark by John Metcalfe, p 64.
Then the book goes into John's baptism of Jesus, showing that it was not a baptismal formula at that time which mattered, but a baptismal foreshadowing signified by going under the waters of baptism, then the Holy Spirit appearing, God addressing his Son, and Christ immediately being driven by the Holy Spirit into the wilderness to face the Adversary:
"The baptismal foreshadowing of Christ's death, resurrection, ascension, and glory, heralded with absolute confidence the redemption which he should accomplish when his ministry on earth closed before that ministry had properly begun. The heavenly witness at Jesus' baptism prejudged the issue of what now lay before the Son in order that he should bring in the gospel." (Ibid. p64)
This should serve to put the question of any baptismal formula John used into its proper place. The Bible mentions none. Had there been one, it was not, in and of itself, significant enough to warrant mention, not least because John's baptism was unique, only applicable to those people at that time, before Jesus began his ministry. It is the baptismal formula Jesus commanded that is so significant that it has been employed by Christians for nearly 2,000 years now. But John did not use it for it had not been stated, nor was it time, during John's lifetime, for Jesus' formula to be used.
What baptismal formula did John the Baptist use?
We simply do not know with any exactitude if St. John used a baptismal formula or not.
11 “I baptize you with water for repentance. But after me comes one who is more powerful than I, whose sandals I am not worthy to carry. He will baptize you with[c] the Holy Spirit and fire. 12 His winnowing fork is in his hand, and he will clear his threshing floor, gathering his wheat into the barn and burning up the chaff with unquenchable fire.”
13 Then Jesus came from Galilee to the Jordan to be baptized by John. 14 But John tried to deter him, saying, “I need to be baptized by you, and do you come to me?”
15 Jesus replied, “Let it be so now; it is proper for us to do this to fulfill all righteousness.” Then John consented.
16 As soon as Jesus was baptized, he went up out of the water. At that moment heaven was opened, and he saw the Spirit of God descending like a dove and alighting on him. 17 And a voice from heaven said, “This is my Son, whom I love; with him I am well pleased.” - Matthew 3:11-17
There seems to be no known tradition from the Early Church on this subject matter. But that does not mean that the Precursor baptized in silence, which in my humble opinion is doubtful.
He certainly did not use a Trinitarian formula. When John the Baptist baptized Jesus, what would the baptismal formula have been? He certainly did not use a Trinitarian formula. The traditional form in the name of the “Father, Son, and Holy Spirit” was not yet in usage. Christ’s redemptive act on the Cross had not yet happened.
It is important, nevertheless, to understand that John wasn’t presiding over conversion rituals. Matthew seems to imply that his spoken baptismal formulation may have been: “I baptize you with water for repentance.” His innovation was an invitation to take up a practice of repentance from sin. Thus they were challenged to prepare for the Messiah’s coming by repenting of their sins and making a new commitment to holiness, symbolized by the ceremonial washing.
A baptism with in silence and no spoken baptismal formula would have mixed meanings.
Even Naaman was commanded to wash by the Prophet Elisha to wash in the waters of the Jordan in order to be healed of leprosy (2 Kings 5). St. John most certainly employed some phrase in his numerous baptisms that he presided over as a Baptist and a Prophet. A true Prophet of the Almighty speaks on his behalf. Remember what Our Lord said of John the Baptist:
Truly I tell you, among those born of women there has risen no one greater than John the Baptist. Yet even the least in the kingdom of heaven is greater than he. - [Matthew 11:11](https://biblehub.com/matthew/11-11.
It seems most plausible that John the Baptist simply said: ” I baptize you with water for repentance.” This would do away with any idea of some form of ritual cleaning that the Jews were accustomed to do!
The Gospels’ main focus is on the ministry of Christ himself, thus what John the Baptist may or may not of said while bailing people was of lesser importance than the redemptive activities mentioned in the Gospels. In any case, the humility of John the Baptist is well know:
29The bride belongs to the bridegroom. The friend of the bridegroom stands and listens for him, and is overjoyed to hear the bridegroom’s voice. That joy is mine, and it is now complete. 30He must increase; I must decrease. 31The One who comes from above is above all. The one who is from the earth belongs to the earth and speaks as one from the earth. - John 3:29-31