FOR a Jew, the great painter Marc Chagall was intriguingly obsessed with the person of Christ. The exhibition "Chagall: Modern Master", at the Tate in Liverpool until next month (Arts, 9 August), immerses the visitor in a dream-world of love and cruelty, birth and death, myth and magic, in which floating figures, symbolic shapes, and strong, emotive colours conjure a new kind of psychic reality.

Chagall was born Moyshe Shagal in 1887 in Vitebsk, a city in what is now Belarus, where Jews, banned from key parts of Russia, were tolerated. Vitebsk's 60,000 inhabitants were split almost equally between Christians and Jews, and that balance had a significant impact on Chagall's formation as an artist. The division created in him not a dichotomy, but an enriched ambiguity.

One of the first pictures in the exhibition is Birth, painted in 1910. Chagall, the eldest in a large family, was around for the birth of all seven of his siblings. The painting depicts a scene in a Jewish shtetl in which the men wait expectantly for news from the birthbed. But with the cow at the back of the room, wise men at the door, and the father secretly present at the birth, the scene has echoes of the nativity, crossing boundaries in a way that was to characterise Chagall's entire career.

The forcefield of energy which was Chagall is an extraordinary, fantastical, mystical jumble of images - of his native Russian home life, of the woman who was to be the love of his life, and of scenes of Paris where he widened the artistic horizons that had blossomed in Russia. It is a world that pays homage to Orthodox iconography as much as to the influence of avant-garde Western art.

There is no doubt how important his Judaism was: the Hebrew scriptures and the community are constantly represented in his art. He quoted the Torah in Yiddish to the end of his life. But one of the most striking paradoxes is the way that, in a number of paintings, Jewish and Christian images sit side by side, and play off one another. Chagall's wandering Jews move amid a landscape dominated by churches, and suffer beneath the shadows of Christ on the cross.

Chagall painted more than 100 scenes of Jesus and the crucifixion throughout his life. After early allusions, it was absent from his work for two decades until the figure of Jesus made an eerie return in 1930. The painter, on a visit to Berlin, had witnessed an increasing tide of German anti-Semitism, and was seized by a premonition of catastrophe. But it was from 1938, when news of the Nazi concentration camps began to leak through to the outside world, that Christ on the cross became a recurring emblem.

The device was controversial. Many who had approved of this quintessentially Jewish painter, reductively dubbed "the Jewish Picasso", were unhappy - not just at these paintings, but also when Chagall later accepted commissions to make stained-glass windows in cathedrals.

But Chagall was untroubled. He referred proudly to "this little Jewish people who gave birth to Christ and Christianity". He saw in the suffering of the Jews the suffering of all humanity, and the innocent man on the cross as its emblem. His Jesus often wore a prayer shawl for a loincloth on the cross.

His painting War is the most powerful example at the Tate exhibition. But perhaps the best example is the new Pope's favourite painting, White Crucifixion, in which the figure of Christ is illuminated by a shaft of light. "It isn't cruel; rather it's full of hope," Pope Francis has written. "It shows pain full of serenity. I think it's one of the most beautiful things Chagall ever painted." It is why Chagall is an artist of shared humanity. 

Paul Vallely's biography Pope Francis: Untying the Knots is published by Bloomsbury.