According to the Bible, God was in the beginning and was the only God worshiped as such. He revealed Himself to Adam and Noah specifically. At Babel, the people of the earth were divided by language, yet all of them retained some history and understanding of God's interactions with mankind in the world that is recorded in Genesis 1-11. From this, we should expect to find similar rituals that match up to those accounts.
In fact, we find just that. In his book, Eternity in their Hearts, Don Richardson records peoples all over the world with "strange traditions" that mirror the concepts of sacrifice, atonement, etc. The Chinese alphabet also has remarkable ties back to Genesis. So, cultures scattered all over the world do share some commonalities with Christianity and Genesis.
However, the similarities of religion are quite small. To quote Malcolm Muggeridge:
We believe that all religions are basically the same– at least the one
that we read was. They all believe in love and goodness. They only
differ on matters of creation, sin, heaven, hell, God, and salvation
Religions do often have very close similarities when it comes to ideas of morality, and the Bible indicates that God's law is written on our hearts. So, that makes sense.
Religions, however, differ greatly on a number of things, including the nature of God, the Person of Jesus, and what man must do with his sins.
The Nature of God
Christianity has a unique concept of God, which is shared in part by Judaism and to a much lesser degree by Islam. In Christianity, man is created in the image of God, whereas in many other religions, God is created in the image of man. An example of this is Greek mythology, which attributed sexuality and such to divine beings.
The Person of Jesus
Another very significant distinction that makes Christianity unique is its answer to the question, "Who is Jesus?" In some religions, men may become gods, but in Christianity, God became a Man and entered into His own creation.
What Man Must Do With His Sin
Finally, what man must do with his sin is perhaps the most unique aspect of Christianity. In Christianity, we have committed sins, and God is just. Sins must be paid for. However, man may not ever pay his own debt for his own sins. In all other religions, a man must do penance of some sort. The idea that if your good outweighs your bad, then you're heaven bound. This idea, though, leaves sins unpunished, and it's based on man making amends for his own sin--just like Adam and Eve trying to cover over the shame of their own sin in the garden. Just as it was in the garden, so it is now that man may not cover over his own sin.
Christianity is based on God reaching down to man and making a complete payment for man's sins once for all through the death and resurrection of Jesus Christ. It is based on a free gift that makes no demands of penance on the individual. A Christian is expected to do good deeds not out of his own will, but out of the transformation that God has worked in his heart upon his faith in Jesus.
One way to put it is that all other religions bid the lame man walk, whereas Christianity heals the lame man so that he can walk.
As an aside, the Bible is quite unique as it is the only sacred text that dares to set forth many specific predictive prophecies that come true hundreds of years later. The year that Messiah was to come was even specified.
So, the answers to the three questions are what make Christianity unique:
Who is God? He is the transcendent creator of the universe, one God in Three Person, whose image we bear (one person with body, soul and spirit). Men are made in the image of God, but God is not in the image of man.
Who is Jesus? He is the eternal Son of God who entered into creation about 2,000 years ago to fulfill the prophecies recorded in the Old Testament. He died on the cross, rose on the third day, and ascended into heaven as King of Kings and Lord of Lords. Men do not become gods, but God did become a man.
What must man do with this sin? Rather than having to work our own way to heaven in order for our sins to not be paid for, but outweighed by good deeds, Christianity teaches that God reaches down to man and provides a complete payment for his sins. We, therefore, stand before God as sinners who are declared righteous. With our debt paid, no one may lay any claim against us, because the debt is fully satisfied. Christians can have complete confidence in this, whereas followers of other religions seldom have any confidence that their good outweighs their bad.