Physically, a virgin birth was impossible, but spiritually it was possible: God isn’t limited by the laws of the physical universe. God is bigger than that. Mary, however, was limited and she needed God’s intervention to live out God’s will for her life.
For us to accept Jesus as Saviour we must first see Jesus as King, rightly having the authority to offer us salvation on His terms. Sin came about when man rejected God’s authority. Salvation comes about when man accepts God’s authority: when we see Jesus as our King.
Our problem is that we trust our senses more than we trust God. Our tendency is to walk by sight, i.e., to live by our senses and our own human logic. When our sensory experiences tell us that things are out of the norm, our minds conclude, based on that limited information, that we are in danger, that we should react in fear.
At Christmas time, and all the time, Believers (“men and women of God”) are appointed to deliver God’s message, to announce God’s truth, to those whom we are sent; just like Gabriel did.