Church preceded the New Testament, not the New Testament the Church. First there was not a Constitution of the United States, and then Americans, who in the light of that Constitution decided to form a government and a nation. The Founding Fathers preceded the Foundation; so the Mystical Body of Christ preceded the reports written later by inspired secretaries. When finally the Gospels were written, they did not prove what Christians believed, nor did they initiate that belief; they merely recorded in a systematic manner what they already knew. People did not believe in the Crucifixion because the Gospels said there was a Crucifixion; they wrote down the story of the Crucifixion, because they already believed in it. The Church did not come to believe in the Virgin Birth because the Gospels tell us there is a Virgin Birth; it was because the living word of God the church already believed it that they set it down in the Gospels.
Pick up the Gospel of Luke, which was written sometime before the year 67, and read the opening lines: "For as much as many have taken hand to set forth in order, a narration of the things that have been accomplished among us: According as they have delivered them unto us, who from the beginning were eye-witnesses and ministers of the Word: It seemed good to me also, having diligently attained to all things from the beginning, to write to thee in order, most excellent Theophilus, that thou mayest know the verity of those words in which thou hast been instructed" (Lk 1:1-4). Luke did not write to Theophilus to tell him something brand new about someone who died over thirty-four years before. Theophilus, like every other member of the Apostolic Church in the Roman Empire, already knew about the miracle of the loaves and fishes, about the Resurrection and the Virgin Birth. It is similar to this. If we pick up a history book that tells us that in 1914 World War I began, it does not create that belief in us, it just confirms what we already know. So, too, the Gospels set down in a more systematic way what was already believed.
Edit: The Tamuz and Horus has been debunked so often its really not worth going into detail again here. What I find interesting is that the people who tend to subscribe to the Horus-Jesus connection hypothesis also laugh at YECs. Hypocrisy at its finest. If it was Christians attempting to use non-scholarly sources to disprove something established by professional mythologists, historians, or scientists, they would be rightly laughed at and disregarded.