Paul had to believe, FIRST.
This happened before He met Ananias, as you find that He called Jesus...."LORD".
This happens in the heart, and not in the water., and it happened to Paul before he was baptized in water.
Jesus was Paul's -"LORD"< before Ananias talked to Paul about John the Baptist's water baptism for Jews.
So, what you have, is a Jew, Ananias..... who is talking to Paul regarding the Baptism of John the Baptist.
This Jew knows nothing about Spiritual Baptism, as that was taught by Paul, much later.
He also does not know that "faith is counted as righteousness", or anything about "Justification by faith", as these were taught by Paul, much later.
So, what you have here, is a standard situation that some Apostles had also...., and this is the same issue that Ananias has...
Its this........ "you can't know what you dont know, until you know it".
So, Ananias, is talking to Paul about John the Baptist's water baptism for JEWS, as that is all he knows... to tell Paul.
Later, .......Paul tells us that Christ didn't send Him to water baptize.....so, that isn't a contradiction,. that is the Gentile Apostle, understanding LATER that the Grace of God is to be born again by : SPIRITUAL Baptism......not by water.
Ananias didnt yet have this revelation, as Paul was the one who would much LATER bring this revelation to the Church.
Ananias not knowing anything about the Gospel of The Grace of God, is why he is talking to Paul about John the Baptist's old covenant water baptism that was given only to : JEWS.
This is like Peter, in ACTs 10, not yet knowing that Gentiles can be SAVED.
See, when you dont know something, you can't teach it., and you sometimes find this in the NT, and these that are doing something that isn't "church doctrine"....., are doing only what they know to do, until later, when they learned MORE REVELATION.