Reflections on Mark's Gospel - Mark 1:8

Comparison between the Baptist and the Messiah, part 2.

John's ministry is summed up as being preaching and baptising. John does not mention the coming Messiah's preaching ministry, but he does contrast his own ministry of baptism with the coming one.

John's baptism was a water baptism of repentance for (or into) the forgiveness of sin.

The Messiah's is a spiritual baptism - "in the Holy Spirit". The Greek word to describe both baptisms is "εν" (+ dative), which usually means "in". Those who do not practice baptism by immersion could have a problem with the first-glance implications of this, and it may lend support to an immersion view of baptism. But there is real value in focussing, not on the form of water baptism, but on the significance of what it means to be baptised in the Holy Spirit.
In the OT, Yahweh is the one identified as giving the Spirit. John transfers this right to the Messiah.

I cannot see how the people of John's day would have understood the implications of receiving the Holy Spirit, beyond seeing significance the fulfilment of the OT prophecies in Joel (Joe_2:28-32), Ezekiel (Eze_11:19), etc. that link the giving of the Spirit with the time of restoration and renewal.

The effect of John's preaching is to leave a sense of anticipation. The crowds who came to him, who confessed their sin and were baptised, would not have felt like this was the end, the final result that God was looking for. They must have had a sense of anticipation. This ministry was preparatory. They were looking for "the one who comes after me", the "greater one".
And, just like we see the transition in v.4, so we find a similar transition in v.9.