Hugo Gryn's words still startle me: "And I understood a bit of the revelation that is implicit in Auschwitz." Is revelation not linked to Sinai, and not anywhere else - let alone Auschwitz? So what can Gryn's words mean?

Shavuot is the festival of revelation; it celebrates how God speaks to us, as a people and as individuals, now, as well as then in that desert at Sinai. Yet it has become the forgotten festival. It does not possess the great themes of liberation like Pesach or the rich rituals of the Seder; it lacks the fun of succah building and the joy of smelling the etrog. Shavuot is almost devoid of symbols. Maybe that is why we treat it as the poor relation among the chagim. Or perhaps it is because the subject of revelation seems less relevant, or more obscure, to us. We can discuss the meaning of freedom for hours. But how many of us feel comfortable talking about God, or how we hear God speaking?

Still, to the seeker these questions are urgent: what does God want of us today? What is God saying to me now? As Jews, we search for God's voice in life and in the Torah, in order to understand what this very Torah means. We listen to God's speech in the wisdom of our ancestors who, century after century, formed and transformed, created and recreated the meaning of that Torah through law and lore, according to their understanding of God's voice in their specific context in history and culture.

Beyond that, more immediately, the soul "thirsts for God, the living God"; we want to hear the voice of God speaking in our conscience and our heart. We seek God's inspiration so that the commandments to love God, God's world and our fellow human beings cease to be mere injunctions and come alive with impassioned immediacy.

Admittedly, to many that is not what matters: "I'll leave God to more spiritual people; what's important is doing good." But spirituality and ethics are intertwined. Our values ultimately derive from our deepest understanding of the world. What we consider God to be telling us determines what we are inspired to do, from the crazed suicide bomber to the teacher who cares devotedly for every child because she believes that God wants children to be loved.

What we hear God say is therefore of the utmost importance. Perhaps, then, Shavuot should really be thought of as the festival of listening.

It is the way the writers hear God which moves me so deeply in the following two testaments from the Holocaust. It was Yom Kippur in 1944 and Hugo Gryn had been weeping for hours: "I believe God was crying too… People sometimes ask me 'Where was God in Auschwitz?' I believe that God was there Himself - violated and blasphemed. The real question is, 'Where was man at Auschwitz?'"

The idea of God crying is not new; the rabbis describe God as descending to weep over the ruins of Jerusalem. To Hugo Gryn also, God cries not in heaven, not from above history, but from within it. Where people are degraded and murdered, God is violated too. The tears of the victims are God's tears; the voice of the suffering and the persecuted is God's voice too. It was Emanuel Levinas who explored this theology to its limits, teaching that we meet God in our every neighbour and in their face encounter our ultimate responsibility.

The writings of Kalonymus Kalman Shapira - known simply as the Rebbe of the Warsaw Ghetto - from the years 1939-1943 constitute the most remarkable testament to spiritual creativity and courage. On February 14, 1942 he taught that one who connects with God's voice in the Torah "hears the sound of the Torah from the world as a whole: from the chirping of the birds, the mooing of the cows, the voices and tumult of human beings - from all these one hears the voice, the unceasing voice, of God in the Torah".

He draws on the mystical tradition which teaches that God's voice in the world and in the Torah are ultimately one. Hence, how we intuit God addressing us out of creation constitutes part of the essential meaning of Torah. The commandments speak not just in black ink on parchment, but in the sound of the birds and the "tumult of human beings". It is unutterably humbling to think of the Rebbe comforting his followers with the awareness that, deeper than horror and anguish, the voice of God still speaks in humanity and in nature, even through the terrors of the ghetto.

These extraordinary testaments extend our sensitivity to what is meant by "revelation". It is precisely this which Shavuot celebrates. Through spending the night in study, by reading the accounts of God's manifestation in the Torah and Prophets, by celebrating nature and human kindness in the book of Ruth, the festival invites us to be mindful.

At its heart are the simple words "And God spoke". How we hear them depends on us. God's speech is always life-enhancing, the essence of the commandment to love. I am moved by those whose God speaks not only from the heavens, but out of the anguish of human need and in the innocent song of the birds.

God calls to us from out of the most terrible suffering and amidst the most exquisite beauty. Our challenge is to listen.

Jonathan Wittenberg is Masorti's senior rabbi