Simple answer:
The foundation that Christ placed for us is still within all of these denominations:
"Love the Lord your God with all your heart, with all your soul, with all your mind, and with all your strength and love your neighbor as yourself."
"All have sinned and fall short of the Glory of God"
"For God so loved the world that he gave his one and only Son, that whoever believes may have eternal life in him. For God did not send his Son into the world to condemn the world, but to save the world through him."
and believe that John was sent to tell us of Christ here:
"John answered them all, 'I baptize you with water. But one more powerful than I will come, the thongs of whose sandals I am not worthy to untie. He will baptize you with the Holy Spirit and with fire.'"
and all agree that "The Son of Man must be delivered into the hands of sinful men, be crucified and on the third day be raised again."
and:
"if Christ is in you, your body is dead because of sin, yet your spirit is alive because of righteousness. And if the Spirit of him who raised Jesus from the dead is living in you, he who raised Christ from the dead will also give life to your mortal bodies through his Spirit, who lives in you. Therefore, brothers, we have an obligation—but it is not to the sinful nature, to live according to it. For if you live according to the sinful nature, you will die; but if by the Spirit you put to death the misdeeds of the body, you will live, because those who are led by the Spirit of God are sons of God. For you did not receive a spirit that makes you a slave again to fear, but you received the Spirit of sonship. And by him we cry, 'Abba, Father.' The Spirit himself testifies with our spirit that we are God's children. Now if we are children, then we are heirs—heirs of God and co-heirs with Christ, if indeed we share in his sufferings in order that we may also share in his glory."
Since pretty much all denominations believe these core beliefs (see the Nicene Creed... most believe the crux of this as well, with some minor disagreements), the denominations really are all part of the same universal body of Christ.
The reasons for the various denominations is that some denominations became more focused on the rules rather than on the relationship with God, and when changes could not be made to bring the denomination back into focus on faith and relationship with God, certain individuals chose to start other denominations to allow themselves the freedom to worship and practice their faith as they interpreted it from scripture. Both sides of the coin chose to nitpick the details rather than focus on the overall goal of the Church - to spread God's love around the earth and to bring people into relationship with Him.
The real problem is that all denominations seem to have forgotten a key point in scripture:
"For by the grace given me I say to every one of you: Do not think of yourself more highly than you ought, but rather think of yourself with sober judgment, in accordance with the measure of faith God has given you. 4Just as each of us has one body with many members, and these members do not all have the same function, 5so in Christ we who are many form one body, and each member belongs to all the others. 6We have different gifts, according to the grace given us. If a man's gift is prophesying, let him use it in proportion to his[
b]faith. 7If it is serving, let him serve; if it is teaching, let him teach; 8if it is encouraging, let him encourage; if it is contributing to the needs of others, let him give generously; if it is leadership, let him govern diligently; if it is showing mercy, let him do it cheerfully. 9Love must be sincere. Hate what is evil; cling to what is good. 10Be devoted to one another in brotherly love. Honor one another above yourselves." - (Taken from the NIV translation, available online:
http://www.biblegateway.com/passage/?search=Romans%2012:1-1&version=31)
Each denomination is, in effect, thinking more highly of itself than the others, and that should not be so. We are all one body with many different giftings, and Christ is the head of that body. If all of the denominations would just realize that we should work together as one unified Christian body (ie one organization, even though we are not all one denomination), we would see the greatest move of God's hand seen since the original beginnings of the Church in the mid first century.
So, hopefully this explains how we are all alike, so therefor can all be correct in the core of our faith, as well as how we are all different, in that we nitpick the details which are truly not important.
It has been my desire for years now to see the whole of Christianity come together an put aside the petty nitpicking items of faith and just believe in the core of Christianity. The only problem is that each denomination differentiates as to what it considers to be the "core". If we all could agree on something similar to the Nicene creed, and all agree to not worry about the specifics of how we go about worshiping and doing the work, we would become what Christ intended us to be.
Again to say, this is how we are all right (at the very core, we agree on at least this one scripture as the sum of all the laws of God:
"Love the Lord your God with all your heart, with all your soul, with all your mind, and with all your strength and love your neighbor as yourself.") and why we are all different yet still in agreement (we all differentiate on the details).
Sorry to ramble on this. I hope I got my thoughts out clearly enough. Let me know if you would like a better explanation.