This is a famous depiction of the Trinity by the 15th century Italian artist, Masaccio.  His wonderful use of perspective makes it look very three dimensional, even though it’s really painted on a flat wall.

Notice the tomb on the bottom of the work, which likely represents Adam (and all of humanity) and the pictures the fate we all share.  The Blessed Virgin and Saint John are pictured at the foot of the cross, and it’s likely that the other two people below them are those who commissioned the work of art, showing their devotion to Christ in a slightly more humble way.  According to the Web Gallery of Art, “A close-up view of the skeleton in the sarcophagus also revealed the ancient warning, in clear letters: I WAS WHAT YOU ARE AND WHAT I AM YOU SHALL BE.”