This is the "Ascension of John the Evangelist Altarpiece" painted in the Italian north. It was made for the church of San Giovanni Evangelista, in Pratovecchio, Tuscany, probably in the 1420s.
In the centre panel, we see Saint John the Evangelist being raised to heaven by Christ, surrounded by a crowd of saints. The large panels on either side show other important saints to the nuns at Pratovecchio - a strict religious order found mainly in Italy.
This altarpiece would have sat on the high altar and shown the saint to whom the church was dedicated. It's one of the few surviving paintings of this date that might have been commissioned by women - two abbesses - for the use of women.
Note how the saints here are those who were important for the nuns at San Giovanni, and some of them are identified by inscriptions. The top pinnacles show the Trinity and the Annunciation, while below is the descent into limbo, where Christ breaks down the gates of hell and leads souls out of purgatory.
The artist, Giovanni dal Ponte, had a workshop in Florence, about 50km from Pratovecchio, so it's likely that he was assisted by a male advisor. Despite this lack of direct connection to the nuns at San Giovanni, the result is a beautiful and intricate work of art that reflects the values and beliefs of its creators.
Ascension of John the Evangelist Altarpiece hangs in The National Gallery on Trafalgar Square, London — free to enter. Point your phone at any artwork there and audioguide.london plays a free audio guide in six languages — no app download needed.