Jesus was considered divine in the earliest Christian circles. The Gospel of Mark calls Jesus the "Lord of the Sabbath", which is something that would've amounted to blasphemy if the early Christians didn't realize he was God. The Horus/Mithra connections to Jesus have been debunked many, many, many times over. Certainly the Christians in Rome or Egypt would've immediately renounced their faith had they thought that an earlier religion already made these stories, for one. Also, most of the events in Jesus' life were predicted in the Hebrew Bible, mostly the books of Wisdom and Isaiah; and these prophecies pre-date almost every myth of Horus.