In Egypt they have a "son of God" called "Horus". In Christianity "Jesus" is the son of God. There seems to be many similarities, how can this be?

It is difficult to respond to a question without specifics, but what I will do is layout some claims that are published and respond to them.

Was born of the virgin Isis (Mary) on December 25th in a cave/manger with his birth being announced by a star in the East and attended by three wise men.

I have found no reference to a cave/manger -- Frazer [Frazer, J. G. Adonis, Attis, Osiris. 1961, pg.8] has Horus born in the swamps, and knows nothing about a star or Wise Men, of any number.

First, it is true that Egyptian legend has Horus being born to Isis. But where did the trailing name of “Mary” come from? No mention in any Egyptian literature links the name Isis to the name Mary. Isis was also not a virgin. No account of Horus’ birth makes this statement. Isis was not a virgin, but the widow of Osiris, another Egyptian god who conceived Horus with Isis. Horus was supposedly born during the month of Khoiak (Oct/Nov), and not on December 25th, a fact which does not help their claim of marrying the stories of Horus to Jesus anyway because the Bible never assigns a birth date to Christ.

When stories detailing the birth of Horus are examined, there is no star or three kings who come to visit him. Trying to link this to Christianity fails in any event as the account of Christ’s birth in Matthew has magi (wise men, not kings) coming to Jesus with their actual number not being stated.

At age 12 he was a child teacher in the Temple, and at 30, he was baptized, having disappeared for 18 years.

Horus becoming a teacher at age 12 (mimicking Jesus’ account at the temple as a youth) is nowhere to be found in accounts of Horus. The only accounts remotely related to Horus and water are the stories told of Osiris (his father who is sometimes combined in ancient accounts with Horus to form one individual) whose body was cut up into 14 pieces by his enemy, Set, and scattered throughout the earth. Isis supposedly found each part of the body and after having Osiris float in the Nile, he came back to life or became the lord of the underworld, depending on which account is read. In any event, stating that Horus was “baptized” is simply playing fast and loose with Christian terminology and is another obvious attempt to link mythology and the Bible.

He had 12 disciples, two of whom were his "witnesses" and were named "Anup" and "AAn" (the two "Johns"). Egyptian religion scholars know of none of this.

There are not any statements to the effect that he had 12 “disciples.” According to the Horus accounts, Horus had four semi-gods that were followers and some indications of 16 human followers and an unknown number of blacksmiths that went into battle with him.

He was crucified between two thieves, buried for three days in a tomb, was resurrected. None of these three can be found, either.

The claims of Horus being buried for three days and resurrected are not to be found in any ancient Egyptian texts either. There is no mention of a burial for three days and no mention of his physically coming out of a grave in the same physical body he went in with and never dying again. And there is certainly no account of Horus dying for others as Jesus did.

From what I have found in all circumstances is that stories about Horus in comparison to Jesus came in modern times not historical accounts. In reference to the validity of Jesus I would recommend reading the book entitled, The Case for the Real Jesus by Lee Strobel. Also The Case for Christ by the same author. This will help you understand why we trust the validity of what the Bible claims about Jesus.