Into what (εις τ). More properly,

Unto what or

on what basis (Robertson, Grammar, p. 592). Clearly, Paul felt they had received a poor baptism with no knowledge of the Holy Spirit.

John's baptism (το Ιωανου βαπτισμα). Last mention of John the Baptist in the N.T. They had been dipped in other words, but they had not grasped the significance of the ordinance.