He painted sea battles, the labours of Hercules and breathtaking still lifes. But, as a major new exhibition makes clear, it was in his thrilling depictions of the spiritual that the Spanish master showed his true genius Against an impenetrable black ground, the crucified figure looms pale and shining. There’s almost no colour, beyond the trickle of blood on Christ’s feet from the nails driven through his flesh. His head slumps, and his carefully modelled face is at peace (no agony here). But...