Dear John:
There are virgin birth myths in antiquity, but the Virgin Birth of Jesus is not a myth.
Nevertheless, it is a common debate point among the unenlightened and illogical to suggest that because certain elements of Christianity can be found in pre-Christian religions and philosophies that Christianity is just another spin-off from those notions in antiquity.
In a very real sense, and certainly from an ontological point-of-view, nothing pre-dates Christianity. The Church is the pillar and foundation of Truth and NOTHING predates Truth; nothing pre-dates God who IS Truth.
Christianity is merely the fullest expression and revelation of God to His children. It is not an off-shoot of anything, it is the "fullest expression" of the Truth that God always was; the "fullest expression" of God's revelation of Himself to his people.
Ancient peoples, pagan peoples, peoples of other religions also have a small measure of the revelation of God. The knowledge of God is written on the hearts of all people. Knowledge of God has been revealed to all people through His creation as St. Paul makes clears in Roman 1:19-20
- 19 For what can be known about God is plain to them, because God has shown it to them. 20 Ever since the creation of the world his invisible nature, namely, his eternal power and deity, has been clearly perceived in the things that have been made. So they are without excuse.
Creation itself testifies to God (the Christian God). No one has an excuse -- not atheist, pagans, anyone.
Creation testifies to the existence of God, but does not give the details of God's nature, economy, or salvation plan. Naturally, as God is seen in creation, primitive peoples who do not have the advantage of the revelation given by God to the Jews and to Christians, formed notions about God as best they could and saw God in the Sun and the Moon and in other objects of creation. They mistook creation for the Creator. This is still going on today in the New Age movement, witchcraft, pantheism, etc.
These ancient people did the best they could with the knowledge they had about God. As a result tidbits of knowledge emerged that lead these ancient people to develop blood sacrifices, myths of virgin births, and all the variety of images and notions that peak at the truth but that did not come into full understanding until Jesus Christ.
These ancient myths and notions and practices are not pre-dating Christianity, they are imperfect precursors and hints of a future understanding God's economy and salvation plan coming into full bloom in Christianity.
Bottomline is that there is a connection between those ancient pre-Christian beliefs and Christianity. The connection is the fact and reality that there is one True God who created all things. There is but One Artist who creates with one brush and thus we see Him and His truth in all things. The ancient people got a peak at the brush strokes, but did not really understand them. Thus they formed notions based upon what they imagined given they did not have the fuller knowledge that we do. This explains the similarity.
It is not a matter of Christianity as an off-shoot of ancient religions, or of those notions believed by ancient religions evolving into Christianity. It is a matter of ancient peoples trying to understand God (the same God we have as Christians) as best they could and in the process finding tidbits of Truth and little peaks into the nature of God. Those tidbits and peaks were not fully understood or revealed until God revealed Himself more fully to the Jews, and then even fuller still in Christianity. The complete and fullest revelation of God will not be given to us until heaven.
None of this will likely convince your friend, but it is not for you to convince him, it is for you to express the truth to him. Plant the seeds of truth and commit him to prayer. Those seeds will hopefully grow and blossom some day, although you may not see the blossoming; it is enough for you to know that you planted the seeds.
God Bless,
Bro. Ignatius Mary