Crucifixion and Last Judgement diptych - Jan van Eyck (c. 1430)

Even though each panel is smaller than two pieces of paper, one above the other (22.25 x 7.75), the level of detail is incredible - as is the source image (if you have the time to navigate there and zoom in, prepare for amazement).

The left panel shows the crucifixion with Jerusalem in the distant background. The right shows the Final Judgement. The frame contains bible verses from Isaiah (Predicts a messiah coming to Earth to redeem the nations of israel), Deuteronomy (Sermons by Moses before the Israelites enter the promised land), and Revelation (Prophetic visions of the day of Judgement).

Oil painting was still in its infancy when this was painted as part of the Northern Renaissance. This is also believed to be the first painting to feature a detailed, realistic moon.

The part that stands out the most to me is the lower-right side - the meditation on damnation. All the beasts - ranging from comical to legitimately terrifying, the various disembodied human remains, and the level of detail - all that fit on less than a piece of paper. It is a wonder from a time when paintings like this were basically movies. This would draw people into the church like an ad. And it would keep them there, engaged unpacking all the details.