In its content the icon of Christ's Nativity has two fundamental aspects: first of all, it discloses the very essence, the immutable fact of the Incarnation of God; it places us before a visible testimony of the fundamental dogma of Christian faith, underlying by its details both the Divinity and the human nature of the Word made flesh. Secondly, the icon of the Nativity shows us the effect of this event on the natural life of the world, giving, as it were, a perspective of all its consequences./.../
[The figure of Joseph] emphasizes that in the Nativity of Christ "the order of nature is vanquished." He is not part of the central group of the Child and His Mother; he is not the father and is emphatically separated from this group. Before him, under the guise of an old and bent shepherd, stands the devil tempting him. On some icons he is represented with small horns or a short tail. The presence of the devil and his role of tempter acquires a particularly deep meaning in connection with this "feast of recreation". Here, on the basis of tradition, the icon transmits the meaning of certain liturgical texts, which speak of the doubts of Joseph and the troubled state of his soul. This state is expressed in the icon by his dejected attitude and is emphasized by the black mouth of the cave, which sometimes serves as a background to his figure. Tradition, transmitted...by the apocrypha, relates how the devil tempted Joseph telling him that a virgin birth is not possible, being opposed to the laws of nature. This argument, assuming different forms, keeps on reappearing through the whole history of the Church. It is the basis of many heresies. In the person of Joseph the icon discloses not only his personal drama, but the drama of all mankind-the difficulty of accepting that which is '~yond words or reason"---the Incarnation of God.
(From The Meaning of Icons by Leonid Ouspensky and Vladimir Lossky)