Acts 1:5

For John indeed baptized with water; but ye shall be baptized in the Holy Spirit not many days hence.

The new birth is a dual thing, as Jesus said, being both "of the water" and "of the Spirit." The apostles had all been baptized with the baptism of John, hence the mention of it here; and the new birth in the Twelve themselves would be an actual reality upon their reception of the Holy Spirit on Pentecost. It is a mistake to understand the outpouring of the Spirit upon the Twelve (promised here) apart from their having already submitted to John's baptism.

Not many days hence ... That is, within ten days intervening between Jesus' ascension and the pouring out of the Spirit on Pentecost.

A further word regarding the baptism of the Twelve. Chrysostom said, "They were baptized by John"; but even apart from such ancient testimony, the deduction is mandatory from the fact of the apostles having aided in the administration of John's baptism (John 4:2). It is impossible to imagine that they were baptizing others with a baptism to which they themselves had not submitted.[8]