John the Baptist is the link between the Old and the New. he stands in the tradition of the Old Testament prophets – proclaiming a message of repentance (metanoia – radical change) and forgiveness.
John’s prophecy – that someone will baptise with the Holy Spirit – is referred to later in Acts 1:5 by Jesus after the resurrection. The baptism that will take place at Pentecost, with the coming of the Spirit.
So what Mark is looking forward to in these words of John ‘He will baptise with the Holy Spirit’ – is beyond Christ’s death and resurrection, all the way to the coming of the Spirit.
Mark is setting his stall out in its entirety. This is what it’s all about in the end, he is saying. The fulfilment of God’s plan through Jesus, that will culminate in the coming of the Spirit.