The Christian faith says that Mary is only a servant of God, yet the same faith proclaims her as the mother of God? Does this title not make the Christian teaching about God into something similar to the concept of trinity known in the old Egyptian religion?
Please start by reading our explanation Mother of God in the above section 8, answer to questions 71 and 72, in particular part 2 of that answer. The Catholic Catechism for Adults (Katholischer Erwachsenen-Katechismus, Bd. 1: Das Glaubensbekenntnis), p. 171 adds:
“Luke’s annunciation narrative describes more clearly what this state of being a mother [Mary] means: not only mother of Jesus and mother of the Lord, but mother of the Son of God. “The Holy Spirit will come on you, and the power of the Most High will overshadow you. So the holy one to be born will be called the Son of God” (Lk 1:35; cf. Gal 4:4). This statement finds its resonance in the Old Testament narrative according to which God’s glory moved before Israel in the form of a pillar of cloud (cf.) Ex 13:21), indeed dwelt in the midst of Israel in a holy tabernacle (cf. Ex 40:34). The cloud is a symbol of the powerful presence of God in the midst of his people. When in the New Testament Mary is overcome by the Spirit of God, she becomes Gods new dwelling place, the new tabernacle of the Covenant in which God made his dwelling among us (cf.) Jn 1:14)
On the basis of this kind of biblical statement the Church at the third ecumenical Council, the Council of Ephesus (431), taught: Mary is the mother of God. All Christians make this confession. The reformers of the 16th century retained it. However, the term “Mother of God” must not be misunderstood. Of course, Mary did not give birth to God as God. That would not be the true teaching of the Gospel but pure mythology with images of a female principle of the godhead and sometimes of quarternity (four gods). Mary, as she is witnessed in the Bible and believed by the Church, remains a created human being! She did not give birth to God as God but to Jesus Christ in his humanity that was united with God in its nature. Thus, the confession that Mary is the “Mother of God” is ultimately follows from the confession that Jesus is the Messiah, the Christ, “true God and true human being” in one person. When the Church adores Mary as mother of God it glorifies Jesus Christ, who in his nature is mediator between God and humanity.