MANY of those who have been reflecting on the great gift to the Church of the life of Pope Emeritus Benedict have identified the centrality of the Trinity in his teaching. The doctrine of God, who is Father, Son, and Holy Spirit, who is self-revealing in self-transcending relationship, is what compelled Benedict to challenge the secularism of post-war Europe.

He repeatedly calls his hearers back to the centrality of Jesus Christ, in whom we find the truth of the mystery of salvation: what it means to be human; what it means to be drawn into relationship with the Father; what it means to live life, as co-creators, in the power of the Holy Spirit.

One work that demands that we attend deeply and persistently to the central tenet of the Christian faith - the revelation of God as Trinity - is the large mural by Hans Feibusch at the east end of St Alban's, Holborn.

At a symbolic but surface level, there is something very powerful about the fact that Feibusch, a German Jew who fled to England in 1933, should be the person who depicts - in a church bombed during a tumultuous conflict with those who would have imprisoned and gassed him - the mystery of salvation, in which his religious history and ours find an ancient narrative of common origin, but also of bloodguiltiness and savage hatred.

THE church that contains the mural was built as a Tractarian statement about the mystery of salvation in the midst of the slums of Victorian London. Today, our challenge is the urgent question of Christian apologetic - telling the truth about humanity redeemed.

Pope Benedict's 2006 Christmas message took us to the heart of the issue, declaring that "without the light of Christ, the light of reason is not sufficient to enlighten humanity and the world". It is only as those who, in Jesus, find the recovery of our God-likeness, participation in the life of the Trinity, that we can understand the dignity of our making, the necessity of our responsibility to and for each other, and the glorious hope of judgement and salvation.

One of the things that I have always been captivated by in Feibusch's mural is the swirling mass of angels, who seem to emanate from and return into the upper, immanent triangle of the Trinitarian life, in which unapproachable light limits our ability to see the face of God.

In graphic image, it captures brilliantly what St Thomas Aquinas says about angels and mortals: angels are more to the image of God, because they perfectly do God's bidding, unencumbered as we mortals are by embodiment or freedom of will.

But in the Archbishop of Canterbury's Lent Book, Abiding (Bloomsbury, 2012), Ben Quash reminds us of the importance of relationships as the basis of how we human beings learn to be ourselves: mortals in the image of God. This is the point at which we might begin to see why Aquinas went on to say that humans reflect God's relational qualities, as Trinity, more deeply than the angels ever could.

As Quash observes: "Human beings, struggling with the challenge of being 'us' [that is, being in the complexity of frequent messiness of our relationships], are in fact communing with an aspect of God's nature that is kept back from the angels."

Our destiny is not to remain as observers. Astonishingly, the mix of beauty and destruction that constitute our social dealings with one another indicate that we are capable of being called into a new and transformative relationship with God, exemplified by participation in a meal: "Today, salvation has come to you, to this house" (Luke 19.9).

The urgent task that now confronts us all is not so very far from that of the Victorians. It is so to live and to engage, that those who have never prayed will freely seek to enter our churches. They will ask us to kneel humbly with them and patiently to teach them what it means to live and pray the words our Saviour gave us: "Our Father . . .".

Dr Martin Warner is the Bishop of Chichester. This is an edited extract from a sermon preached at the 150th anniversary of St Alban's, Holborn.