Why did the Christians fear God?

People feared God because they thought he was extremely powerful. Many of the churches said that the wrath of God was unbelievable. It was said that on judgment day if you had sinned God would punish by having you die and sent to the underworld.

But many people survived on judgment day therefore of the first crusade.

-History Teacher at Dalton School

Answer

The fear of God came from the imagination that he was a very temperamental and rigid personality who demanded slavish supplication because alternatively he would unleash his terrible and unconquerable wrath upon the helpless innocent creatures of the earth. From time to time he would demonstrate his terrifying prowess by unleashing merciless floods, tsunamis, storms, earthquakes, fiery brimstone, droughts and plagues so that his slavish human pets would maintain absolute obedience and loyalty to his governance. God was therefore a terrorist leader who use his tremendous power to dominate and control the world.  

Wrong religion, Wrong God, Wrong word

The question itself is rather leading in that it implies Christians are scared of a nasty god who will punish at the drop of a hat. Nothing is further from the truth.
Firstly, historically the gods of Babylon, Assyria - and even, more recently, the Aztec Empire in Mexico, as well as many others were very much regarded as ogres in the sky that had to be appeased if things were to go right. Sometimes with devastating ceremonies involving human sacrifice. However the Jews believed in a God that was to be respected, but not feared in the modern sense of the word. Any sacrifice was of animals, swiftly and humanely slaughtered (compare this with a modern abbatoir) and most of the meat given to the poor, the remainder given to the priest as his sole payment or burned on the Temple altar. Human life was sacred. Despite the rules, though, the Jews rebelled and continually found loopholes that enabled them to live life as they wanted rather than as God wanted.
The problem here is with language. The King James Version of the Bible, good though it is, still uses archaic words that have changed their meaning today. Modern translations now translate the Hebrew word that was translated as 'fear' in the KJV nowadays as 'respect' or 'awe' as this is nearer to the original Hebrew meaning - because 'fear' although once meaning 'respect' now means nearer to being 'scared' or 'frightened'. As just one example of the attitude of God in the old testament, just read a part of the prophesy of Isaiah (here in an ultra-modern translation), where God speaks through Isaiah to the Jewish people:
Is 43: 1-7
But now, God's Message, the God who made you in the first place, Jacob,
the One who got you started, Israel:
"Don't be afraid, I've redeemed you.
I've called your name. You're mine.
When you're in over your head, I'll be there with you.
When you're in rough waters, you will not go down.
When you're between a rock and a hard place,
it won't be a dead end-
Because I am God, your personal God,
The Holy of Israel, your Savior.
I paid a huge price for you:
all of Egypt, with rich Cush and Seba thrown in!
That's how much you mean to me!
That's how much I love you!
I'd sell off the whole world to get you back,
trade the creation just for you.
"So don't be afraid: I'm with you.
I'll round up all your scattered children,
pull them in from east and west.
I'll send orders north and south:
'Send them back.
Return my sons from distant lands,
my daughters from faraway places.
I want them back, every last one who bears my name,
every man, woman, and child
Whom I created for my glory,
yes, personally formed and made each one.'"
Hardly the words of a God to be scared of!
As for Christianity, Christians have never been taught to be scared of God. Jesus made that very clear. A healthy respect, yes, but not fear. He taught that we should regard God as an ideal parent - loving and giving, someone to turn to when we are in need, and someone who may well have to keep us on the straight and narrow if needed, but out of love. And no matter what we do, that parental love will always be there, unconditionally. If we do stray, then we will always be welcomed back with open arms, if we turn back to him. In fact Jesus taught us to call God "Abba" - an Aramaic phrase (Jesus' native language was Galilean Aramaic) that roughly translated into English, means "Daddy".
Hardly the term used for a God whom we should be scared of!  

The Word "Fear":

When the the word "fear," in respect to God, is used in Scriptures, it generally has the connotation of reverence. In the Old Testament, the Hebrew word most often used is yare, meaning "to revere." In the New Testament, the Greek word or a root of the word for fear is phobeo or phobos, from which we get "phobia" or fear. However, phobosmeans "reverence" or "fright", phobeo is "to revere," or passively "to be frightened."
The basic idea in fearing God is a wholesome fear. This may be compared to an electrical outlet. One would not, in sanity, remove the cover plate and stick their finger inside where the bare connections are. Yet one is not reluctant to insert a plug into the outlet to use the electricity properly.
A wholesome fear of God will amount to not to disregard His good instructions and to obey His commandments, as John says: "His commandments are not grievous." 1 John 5:3. Also, it is wholesome to be afraid to violate His instructions, knowing that the consequence could issue in harm to our well-being, and loss of God's blessings.
"Let us hear the conclusion of the whole matter: Fear God, and keep his commandments: for this is the whole duty of man." Ecclesiastes 12:13.