The qualification for the new apostle that began with the words "It is necessary ... that one of the men who accompanied us" in verse 21 ends here. The subject of the verb "become" is thus "one of the men." Here is a reduced form of the sentence: "It is necessary ... that one of the men who accompanied us ... beginning from the baptism of John ... must become a witness with us."
The noun "baptism" can be translated as a verb. Possible meanings: 1) "beginning from when John baptized Jesus" or 2) "beginning from when John baptized people" (See: [[rc://en/ta/man/jit/figs-abstractnouns]])
This can be stated in active form. Alternate translation: "until the day when Jesus left us and rose up to heaven" or "until the day that God took him up from us" (See: [[rc://en/ta/man/jit/figs-activepassive]])