I tend to agree that the Jesus-Horus thing is overstated. There are plenty of similar myths to borrow from, and the concept of rebirth/resurrection was well established in various cultures of the time. The more direct connection is through Mithraism
"Both Mithras and Christ were described variously as 'the Way,' 'the Truth,' 'the Light,' 'the Life,' 'the Word,' 'the Son of God,' 'the Good Shepherd.' The Christian litany to Jesus could easily be an allegorical litany to the sun-god. Mithras is often represented as carrying a lamb on his shoulders, just as Jesus is. Midnight services were found in both religions. The virgin mother...was easily merged with the virgin mother Mary. Petra, the sacred rock of Mithraism, became Peter, the foundation of the Christian Church."
Because of its evident relationship to Christianity, special attention needs to be paid to the Persian/Roman religion of Mithraism. The worship of the Indo-Persian god Mithra dates back centuries to millennia preceding the common era. The god is found as "Mitra" in the Indian Vedic religion, which is over 3,500 years old, by conservative estimates. When the Iranians separated from their Indian brethren, Mitra became known as "Mithra" or "Mihr," as he is also called in Persian.
Mithraism was practiced by the Roman soldiers. It is an easy connection to make, and the similarities are striking.