For all its holiness, the Vatican of the 16th century was a deliciously petty affair, riddled with gossip, intrigue and enmity. Michelangelo had a particularly awe-inspiring array of enemies. He attacked Pope Julius II with a plank while working on the Sistine Chapel ceiling, and then painted a thinly-veiled portrait of the Pontiff sworn at by a cherub. Pope Leo X called the artist 'terrible'.

Raphael was another enemy; theirs was a bitter rivalry. In order to topple Raphael's career, Michelangelo promoted the rather ordinary young Venetian Sebastiano del Piombo ahead of him. Sebastiano and Michelangelo's creative partnership is the basis of an enormous, academic and rather wonky National Gallery exhibition that highlights the yawning gap between the genius and his acolyte.

The exhibition encompasses 70 works – paintings, drawings, sculptures and letters – produced by Michelangelo and Sebastiano before, during and after their association. We have a wealth of letters that give a remarkably intimate glimpse into their personal and professional lives.

You leave the exhibition struck afresh by the sheer scale of Michelangelo's genius. Somehow, the National Gallery has managed to bring his white marble Risen Christ to London – it hasn't moved from San Vincenzo Martire monastery in Bassano Romano, a village just outside Rome, since 1644. It is an astonishing work: The naked, muscular humanity of Jesus are as one with his divinity. He is both saviour and carpenter, both god and man. You gasp when you set eyes on it.

We have a cast of Michelangelo's desolating Pieta, in which the Virgin cradles her dead son, diminished by her grief. (It's a bit of a shame we don't get the real thing).

Sebastiano's paintings cannot stand up to these majestic works. Some of them are laughably bad – a cherub with the face of an old crone is fairly terrifying. His colours are garish, his compositions fall flat. The younger Venetian is simply no match for Michelangelo Buonarroti, hero of the High Renaissance.