This is "A Young Man Praying" painted by the Master of the Holy Blood. The subject of this painting is Martin Luther, a leading figure of the Protestant Reformation in his time. This portrait was originally part of a larger work, likely a folding diptych or triptych. If you look closely, you can see remnants of red paint on the reverse side, which may have created a dramatic effect by showing an object against a red background.
The style of this painting is reminiscent of Bruges paintings from the 1520s. The Master of the Holy Blood was known to distort his subjects' faces in a particular way, and you can see that here - the nose is almost in profile, while the face is turned in three-quarter view. It's likely that this painting came from his workshop, although it's difficult to say for certain due to its condition.
A Young Man Praying 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.