The Emperor Theodosius (about 346 - 395) - in armour and a laurel wreath - is prevented from entering Milan Cathedral by Saint Ambrose (about 340 - 398), bishop of the city. The saint had banned Theodosius from the cathedral after the massacre of a subject population in Thessalonica. Van Dyck has identified one of Theodosius's companions as Ruffinus, whose shameless arrogance Saint Ambrose compared to a dog (which is at his feet). The story of Saint Ambrose is related in 'The Golden Legend'.
This picture is a 'free copy' by Van Dyck of a larger picture (Vienna, Kunsthistorisches Museum) of about 1618. The painting was designed by Rubens and executed in his studio, probably by Van Dyck in his 'Rubensian' style. This work, which is in a far freer style, was probably painted about 1619-20.
The second man from the right is a portrait of Nicolaas Rockox (1560 - 1640), the Antwerp patron who commissioned 'Samson and Delilah' from Rubens and may well have commissioned this picture too.