Matthew 21 was a clever way of Jesus exposing the Jewish leaders' intentions. He asked them if John's baptism came from God. If they answered yes, then this would have validated Jesus' ministry. If they had answered no, then the people would have been upset.
With John's baptism, if you are claiming it was a Christian baptism, then my response would be, why were those who experienced John's baptism baptized AGAIN in Acts 19?
John's baptism was to impress upon Jews that they were not clean by virtue of being physical descendants of Abraham. They were required to wash themselves to show that they were not. This was preparatory to the coming of Jesus. It demonstrated the humility and admission of guilt for their sins.
And, if it was a Christian baptism, then you have a problem because those baptized by John were re-baptized in Acts 19.
Why? They didn't even know Jesus' name, so how could they identify with him? They knew, through John's teaching, that they were not justified due to their identification with Abraham, but they didn't know that they needed to be identified with Jesus.
Acts 19:1-7 1 And it happened that while Apollos was at Corinth, Paul passed through the inland country and came to Ephesus. There he found some disciples. 2 And he said to them, “Did you receive the Holy Spirit when you believed?” And they said, “No, we have not even heard that there is a Holy Spirit.” 3 And he said, “Into what then were you baptized?” They said, “Into John's baptism.” 4 And Paul said, “John baptized with the baptism of repentance, telling the people to believe in the one who was to come after him, that is, Jesus.” 5 On hearing this, they were baptized in the name of the Lord Jesus. 6 And when Paul had laid his hands on them, the Holy Spirit came on them, and they began speaking in tongues and prophesying. 7 There were about twelve men in all.
(ESV)
That is why the "name of Jesus" is mentioned here. It isn't because a Triune baptism is inappropriate. It is because they needed to be identified with Jesus, the Savior who bought them.
We know from Matthew 28:18-20 that Jesus had no issue with a Triune baptism. This is something that anti-Trinitarians cannot acknowledge. They must clasp their hands over their eyes when it comes to that verse, and either ignore or deny it.
JESUS HIMSELF SAID IT, THEREFORE IT IS TRUE. AND THOSE WHO DENY IT ARE TEACHING FALSELY.
Matthew 28:18-20 And Jesus came and said to them, “All authority in heaven and on earth has been given to me. 19 Go therefore and make disciples of all nations,
baptizing them in the name of the Father and of the Son and of the Holy Spirit, 20 teaching them to observe all that I have commanded you. And behold, I am with you always, to the end of the age.” (ESV)
Notice, I have no problem with the claim that baptism is a command. I do have problems with those who claim that baptism causes salvation, because this is false. God regenerates the person, causing him to respond in faith and repentance, and that is what salvation is about.
Ephesians 1:13-14 13 In him you also, when you heard the word of truth, the gospel of your salvation, and believed in him, were sealed with the promised Holy Spirit, 14 who is the guarantee of our inheritance until we acquire possession of it, to the praise of his glory. (ESV)
And, again, I will point to Acts 10, where Cornelius and his family were saved before they were baptized. A Oneness Pentecostal should be able to see that this is true, since they claim tongues are a necessary evidence of salvation, and they were speaking in tongues prior to baptism.
I don't agree with their claims regarding tongues, including the fact that tongues are not gibberish but real languages in the book of Acts, but even using their logic, they should be able to see they are being inconsistent by claiming salvation occurs at water baptism.
By the way, I changed my mind and decided to respond to you anyways. I'm fickle that way