A couple things you are confused about...
1) I would not claim to know the existence of God, or veracity of the Bible with absolute scientific certainty. I can tell you that there are ALWAYS other possibilities, as have been proven many times before... even during progression of Christianity, and conception of Adventism.
But the point is that based on the contrary evidence I've seen, the claims that Horus' story parallel's Christ to the point described are bogus at best. But as Trent Reznor would sing, Ironically:
"So naive, I keep holding on to what I want to believe..."
I've heard the stuff you proposing before I was even a Christian... yet much of it is by people who want to sell books...
For example, there's a fellow named David Icke, he's a firm believer that Christianity is an agglomeration of stories from the past that is used for control. But then he also thinks that this world is run by shape shifting reptilians from other dimensions. I guess the pouring dirt on Christian history alone does not sell enough books :).
But, if you really take your time and do thorough research, and look at reputable resources, like Encyclopedias and etc... you wont find much evidence that "Zeitgeist" films are pushing. I too at one point found it the info fairly interesting, but if you dig and check the facts... THEY DON'T ADD UP!
For example:
Claims about Horus:
Was born of the virgin Isis-Meri in December 25th in a cave/manger with his birth being announced by a star in the East and attended by three wise men.
At age 12 he was a child teacher in the Temple, and at 30, he was baptized, having disappeared for 18 years.
Was baptized in the river Eridanus or Iaurutana (Jordan) by "Anup the Baptizer" (John the Baptist) who was decapitated.
He performed miracles, exorcized demons and raised El-Azarus ("El-Osiris") from the dead.
He was crucified between two thieves, buried for three days in a tomb, was resurrected.
The Lord's Prayer was prefigured by an Egyptian hymn to Osiris-Amen beginning, 'O Amen, O Amen, who are in heaven.' Amen was also invoked at the end of every prayer
... And many more...
Can you find me any reputable source that would support these claims, because I sure can't?
The only even remote resemblance that I could find is the images of the Mother/Child, and the concept of deity called "Shed" and the concept of "Shed the Savior", yet he WAS NOT the official deity in Egypt. He was a god that would save them from illness or danger.
And the more I dug, the more I realized the these claims are made by seude-egyptologists that are looking to push fiction to "hold on to what they want to believe"... especially the claim that Horus had 12 disciples and died on the cross. Can you PLEASE find me some reputable source, from a reputable Egyptologist that can confirm these claims, because I sure can't, and I've been searching for a while.
Egytian "Anch" for you. While I agree that the similar cross has been used by the pagan religions for centuries, again... to support your view you dismiss the possibility that it was the pagans who used it based on prophesies about Christ. You say you are agnostic, yet you absolutely dismiss this possibility? Why? Should I sing the song again?
On top of everything, I don't necesserily believe that Jesus was "crusified". The word "stauros" that's translated as "cross" today, really means "Steak" or "Pole". And further writers refer to the object as "the tree". So, there's no absolute certainty that the object was indeed the cross... but my point is that the object really makes no difference or is essential to the Christian belief as it was made by pagan culture.
That's why I particularly don't like the images of the cross because they give a warped view of Christian belief and it all becomes about death and suffering, instead of joy and life.
Either way, I do respect your ideas, and I do understand where you are coming from. I don't know what's your story and experience is, but why are you "proving" things to people who you know that song was written about?