On the first Sunday of his visit to India, the Archbishop of Canterbury preached at a service of thanksgiving to mark the Global Day of Prayer for the millennium development goals at St Paul's Cathedral in Kolkata.

Most of the time, we try to make reality fit our preferences and our self-directed hopes. But the Spirit will not leave us in peace with our own version of reality; the Spirit brings us into God's world, God's frame of reference. When we say that Jesus was filled with the Spirit, what we mean is that Jesus lives every moment in God's world, not in the confused muddle of fears and hopes and self-serving preferences that the rest of us inhabit for so much of our lives.

And when that reality is uncovered, what do we see in the world when we are living in God's frame of reference? We see a world where people are not free, where they are denied a future, where they cannot themselves enjoy a truthful and hopeful vision of things. Our selfishness and untruthfulness have the effect of blocking out vision for other people and leaving them in the dark, unable to grasp the choices and possibilities they actually have. The Spirit is thus the power that brings liberty to prisoners and sight to those who cannot see. And we who live in the Spirit are committed to that agenda – to release and to the vision of hope.

The prophet Isaiah speaks of the "fast" that God wants; and we can understand this a bit better in the perspective of this vision of the Spirit. Real fasting, says God to the prophet, is breaking the bonds of injustice and sharing your resources. And it is fasting because it means denying yourself something – not denying yourself material things alone, as in the usual sort of religious fasting, but denying yourself the pleasures of thinking of yourself as an isolated being with no real relations with those around; denying yourself the fantasy that you can organise the world to suit yourself; denying yourself the luxury of not noticing the suffering of your neighbour. This is fasting that reconnects you with reality. And in the context of the gospel, this is the fasting that the Holy Spirit makes possible for us, breaking through our self-satisfaction.

All this makes very plain the strange fact that our world today has more rapid communication than ever before – and yet we seem less able to see and to face the reality that's there in front of us. So much of our national life in various contexts is devoted to protecting the fantasies and the denials. Not only individuals but whole nations can behave as if they were alone in the world or as if they could shape the world according to their own agenda. Through the centuries, and in some parts of the world today, oppressive and sometimes brutal governments have worked to keep themselves in power while turning their own nations into paupers; they refuse to see what is literally in front of their eyes. In many contexts, you will see societies dealing with their shame over injustices that everyone is vaguely aware of by avoiding anything that brings into the light the actual scale of suffering involved.

When we pray for justice for the poor and for the whole of our material environment also, we are really praying for the gift of the Spirit to open our eyes and help us to "fast", to turn away from the unhealthy diet of falsehood. And we pray, therefore, for the pouring out of the Spirit upon "all flesh" – not just on believers. For Christians seeking to serve God's agenda, there must be willingness to work with anyone whose eyes are open to the reality before them; they too have received some portion of the Spirit.

• The full version of Rowan Williams's sermon can be found at the Archbishop of Canterbury's website