This is a fresco by Masaccio (an important Renaissance artist) of the Holy Trinity. It is a landmark work of art, considered the first successful illusion of one-point perspective. It ignored detail and focused on the realistic qualities of mass and depth. It shows Christ (the Son) on the cross with the Father behind. The Trinity is made complete with the presence of the Holy Spirit. (Can you find Him?) Masaccio went with the idea that the Holy Spirit took the form of a dove (as with our Lord's baptism), and so the Holy Spirit is the little white dove that actually looks like God the Father's neck collar. The people on the left and right are the patrons who commissioned the work (their placement at the foot of the cross would demonstrate their devoutness—and by having a massive fresco painted of them it actually was sending the message of how humble they were…yeah, got that).
Shortly after Masaccio's Holy Trinity, Brunelleschi developed his theory of Linear Perspective—a geometric method of representing the way that objects appear to get smaller and closer together the farther away they are. The first book to include a treatise on Perspective was published in 1436.
One-point perspective will dominate art until we get to Impressionism. To mimic a 3-dimensional environment it contains a vanishing point, which is the point (often invisible) where the floor touches the sky—the horizon line, basically. One-point perspective, naturally, contains one vanishing point (the meeting point of all angles). Arguably the most definitive painting of one-point perspective was painted during the Renaissance, to be found on a wall in the Sistine Chapel in Vatican City.