Joseph was naturally afraid at the novelty and splendor of the miracle and the depth of the mystery. We need not wonder that he thought himself unworthy of the society of such a virgin when we hear the holy Elizabeth exclaim with fear and trembling : “Whence is this to me that the mother of my Lord should come to me?”
That is why Joseph wanted to send her away. But why in secret and not openly? To avoid any investigation of the reason for the separation, which required an account of it.
If he had told his opinion and the proof he had of Mary's purity, would not the people have mocked him and stoned Mary? How would they have believed in the Truth still mute in the womb? What would they not have done to the still invisible Christ? It is therefore with good reason that this just man, in order not to be reduced to lying or exposing an innocent woman to blame, wanted to send Mary away in secret.
Saint Bernard, excerpt from the 2nd sermon “Super Missus est”