With one another.
‘If we say.
He will say.
Did you not
καὶ διελογίζοντο πρὸς ἑαυτοὺς λέγοντες Ἐὰν εἴπωμεν Ἐξ οὐρανοῦ, ἐρεῖ Διὰ τί οὖν οὐκ ἐπιστεύσατε αὐτῷ;
This argument among the Jewish leaders can be found in Matthew, chapter 21:25, and Luke, chapter 20:5, almost word for word. Mark said that the high priests, Scribes, and the elders argued or discussed with each other (καὶ διελογίζοντο πρὸς ἑαυτοὺς). If they said that his baptism was from heaven (λέγοντες·Ἐὰν εἴπωμεν Ἐξ οὐρανοῦ), then Jesus would ask them why they had not believed in John the Baptist (ἐρεῖ Διὰ τί οὖν οὐκ ἐπιστεύσατε αὐτῷ)? This was a real option, but one they did not want to take.