The Spirit of Jesus, the Spirit of God, poured out upon us to make us holy, is curiously self-effacing. Trying to describe the Spirit is like trying to catch the wind. It is much easier to point to the effects of the Spirit than to say who it is.
Both St Luke and St John wrestle with this, giving us two pictures of the Spirit's work, which is part of the one great saving act of God in the mystery of Easter. Christians celebrate Pentecost as the end of a fifty-day Easter season, which includes the Resurrection and the glorification (or Ascension) of the Lord Jesus, and the new life of the sons and daughters of God communicated by the Lord by the sending of his Spirit.
Yet the description given us by Luke is sparse: there is a noise, 'like a wind', that fills the house; and there appeared to the disciples tongues 'like fire'. The rest of the reading from Acts describes how the Spirit-filled disciples were perceived by others. Luke is pointing to the spiritual transformation and empowerment of the disciples by using familiar images of God's coming and action: the loud sound recalls God speaking to Moses in thunder at Sinai; the wind God's coming to Elijah and Elijah's ascension; the fire that does not destroy recalls God's calling Moses aside at the burning bush - Pentecost originated from a harvest feast, but eventually came to celebrate the giving of the Law on Sinai; and the Law, which makes humans pure before God, was symbolised by fire.
John, in contrast, links the sending of the Spirit much more closely with the Resurrection. 'On the evening of that day, the first day of the week', the day of the Resurrection, Easter day. 'That day' began with the women finding the tomb empty at dawn, with Mary mistaking the risen Lord for a gardener, and now concludes with the risen Jesus's presence among his cowering disciples. He breathes on them and says 'receive the Holy Spirit'. Jesus now lives - he breathes - and gives to his followers new life, animating them with the life of God, the Spirit, which causes them to act in order to realise God's saving work.
They are the ones who will bring the holiness of Jesus to further generations, even to ourselves, thus enabling the ongoing experience of peace and joy that only faith in Jesus can bring. They will bring God's forgiveness for all sin that is to be forgiven, and lay bare all sinfulness.
Earlier, taking leave of his disciples, Jesus has told them that the Spirit, the Paraclete, will continue the revelation of God in Jesus, and just as Jesus by his presence and actions reveals the truth and light of God that in itself constitutes a judgement on the world, so the Spirit will expose the false judgements of the world and lead us into the truth. The gift of the Spirit universalises the presence of the risen Lord within the worshiping disciples, empowers them to extend God's forgiveness down through the generations. The risen and ascended Lord by the gift of his Spirit transcends his previous physical particularity, as indicated by Mary being told not to cling to him in that mode in the garden.
Our being made holy, sanctified, by the Spirit will bring us to blessedness before God, but this is not necessarily an easy option: the wind of the Spirit will scour and expose all that rejects the love lavished upon the world by a God who has sent his only Son. To refuse belief, to reject the Spirit, results in our self-condemnation. We are invited to let ourselves be drawn from the stasis of our sins and selfishness, to let ourselves go with the wind of the Spirit of God, as he comes to us, is active in us and works through us, drawing us to him in communion and friendship; the presence that makes itself felt in his gifts - wisdom, insight, counsel, power, knowledge and reverence before God - which enable us to show forth in our lives the fruits of the Spirit: love, joy, peace, patience, kindness, goodness, trustfulness, gentleness and self-control.