You keep a very selective set of facts.

Why We Are Afraid, A 1400 Year Secret, by Dr Bill Warner
http://www.youtube.com/watch?v=t_Qpy0mXg8Y

"unless you want to incorporate elements of Mithraism into Christian theology?"

I noticed you didn't consider that the Christians were introducing Christianity into Mithraism which they in fact did and it worked.

In the meantime it doesn't matter what day the Christians picked to celebrate the birth of Christ, because people like you would find some other objection, based on some other event or religion.

There's only 360 days in a year and civilization is thousands upon thousands of years old. I'm sure there has been some kind of tragedy or heinous belief on every one of those days by somebody.

Again, your starting with the premiss that God doesn't exist, so your grasping at any thing to support that position.

The term"Christianity," especially since the Reformation, has covered an
astonishing range of groups. Those claiming to represent "true
Christianity" in the twentieth century can range from a Catholic
cardinal in the Vatican to an African Methodist Episcopal preacher
initiating revival in Detroit, a Mormon missionary in Thailand, or
the member of a village church on the coast of Greece. Yet Catholics,
Protestants, and Orthodox agree that such diversity is a recent—and
deplorable—development. According to Christian legend, the early
church was different. Christians of every persuasion look back to the
primitive church to find a simpler, purer form of Christian faith. In
the apostles' time, all members of the Christian community shared
their money and property; all believed the same teaching, and
worshiped together; all revered the authority of the apostles. It was
only after that golden age that conflict, then heresy emerged: so says
the author of the Acts of the Apostles, who identifies himself as the
first historian of Christianity.

But the discoveries at Nag Hammadi have upset this picture. If we
admit that some of these fifty-two texts represent early forms of
Christian teaching, we may have to recognize that early Christianity
is far more diverse than nearly anyone expected before the Nag
Hammadi discoveries.26

Contemporary Christianity, diverse and complex as we find it,
actually may show more unanimity than the Christian churches of
the first and second centuries. For nearly all Chris-
[ xxii]
Introduction
Christians since that time, Catholics, Protestants, or Orthodox, have
shared three basic premises. First, they accept the canon of the New
Testament; second, they confess the apostolic creed; and third, they
affirm specific forms of church institution. But every one of these—
the canon of Scripture, the creed, and the institutional structure—
emerged in its present form only toward the end of the second
century. Before that time, as Irenaeus and others attest, numerous
gospels circulated among various Christian groups, ranging from
those of the New Testament, Matthew, Mark, Luke, and John, to such
writings as the Gospel of Thomas, the Gospel of Philip, and the Gospel of
Truth, as well as many other secret teachings, myths, and poems
attributed to Jesus or his disciples. Some of these, apparently, were
discovered at Nag Hammadi; many others are lost to us. Those who
identified themselves as Christians entertained many—and radically
differing—religious beliefs and practices. And the communities
scattered throughout the known world organized themselves in
ways that differed widely from one group to another.

Yet by A.D. 200, the situation had changed. Christianity had become
an institution headed by a three-rank hierarchy of bishops, priests,
and deacons, who understood themselves to be the guardians of the
only "true faith." The majority of churches, among which the church
of Rome took a leading role, rejected all other viewpoints as heresy.
Deploring the diversity of the earlier movement, Bishop Irenaeus
and his followers insisted that there could be only one church, and
outside of that church, he declared, "there is no salvation."27
Members of this church alone are orthodox (literally, "straightthinking")
Christians. And, he claimed, this church must be
catholic—that is, universal. Whoever challenged that consensus,
arguing instead for other forms of Christian teaching, was declared
to be a heretic, and expelled. When the orthodox gained military
support, sometime after the Emperor Constantine became Christian
in the fourth century, the penalty for heresy escalated.

The efforts of the majority to destroy every trace of heretical
"blasphemy" proved so successful that, until the discoveries at Nag
Hammadi, nearly all our information concerning alternative forms
of early Christianity came from the massive orthodox attacks upon
them. Although gnosticism is perhaps the earliest—and most
threatening—of the heresies, scholars had known only a handful of
original gnostic texts, none published before the nineteenth century.

http://sanctuaryinterfaith.org/wp-content/uploads/The-Gnostic-Gospels.pdf

I believe the books of the Bible were purposely selected so as to give authority to the church. After reading several of the Gnostic Gospels, I understand why they might have been omitted form the Bible and declared heretical in the attempt to centralize authority.

Several Gnostic passages state that you and I have the power to tap directly into the higher source of knowledge. Consider the two sayings that follow, both attributed to Jesus (followed by the name of the Gnostic text):

What you seek after (is) within you. ~The Dialogue of the Savior

Beware that no one lead you astray, saying ‘Lo here!’ or ‘Lo there!’ For the Son of Man is within you. Follow after him! Those who seek him will find him. ~The Gospel of Mary