There's an excellent post (the first of two) from the NLM on the ancient iconography of the Nativity scene [here]
"...More than one modern writer has failed to grasp or acknowledge the pastoral concerns of men like St. Irenaeus, Origen, or the fathers of the Council of Nicea, when they assailed heresies like Gnosticism or Arianism. In defending the full humanity and the full divinity of Christ, they are also defending the fullness of God’s salvation for the sake of every single person alive, of whatever sort or condition. For the heretics, human nature cannot be saved by the divine nature, because the latter can have nothing in common with it. The faith of the Church can look upon a newborn in a manger and proclaim to all men that in Him, God has made known His Salvation, which is Himself."