The Nativity, one of Petrus Christus' most important devotional paintings, emphasizes the sacrificial nature of Christ's coming and shows the scene as part of a chain of events in the story of the Fall and Redemption of humankind. In the foreground, a sculpted archway displays scenes of the Fall as described in Genesis. Below Adam and Eve, Atlas-like figures symbolize humanity burdened by Original Sin. The artist's depiction of the scene is like an act from a mystery or Passion play, the figures clothed in simple Flemish costume and provided with a landscape backdrop of what, at first, appears to be a Netherlandish town. However, with its two domed buildings, it would be understood as Jerusalem, the scene of the events of Christ's Passion.
Christus depicted not only the historical moment of Jesus' birth but also the enactment of the first Mass, an image deriving in part from the revelation of Saint Bridget, which had become the conventional visualization of the Nativity by the early fifteenth century. The angels wear eucharistic vestments of the subministers of the Mass, though none wears the chasuble worn by the principal celebrant, suggesting that Christ himself is here both priest and sacrifice.
More information on this painting can be found in the Gallery publication Early Netherlandish Painting, which is available as a free PDF https://www.nga.gov/content/dam/ngaweb/research/publications/pdfs/early-netherlandish-painting.pdf