Your best bet is to read the Catholic Catechism. Here’s a link to the Catechism in English:
Catechism
You could also look up the Nicene creed, which contains a very brief summary of the key beliefs of Christianity.
The short version is this. God created mankind (Adam and Eve) in a state of natural perfection, but they rejected Him. This was the Fall, and it brought sin and death into the world. To save us from the effects of the Fall, God first prepared a people (the Jews), who learned something of His nature (one God, not many) and His name, and His plan to send an anointed one, a Messiah, to save His people.
In the fullness of time God sent this Messiah, which turned out to be God Himself, become fully man while fully God. God thus chose to become united with all of His creation in order to redeem all of His creation. This man who was God incarnate was Jesus the Christ (the anointed), who was born in a poor stable in a manger (feed trough) in the town of Bethelem (House of Bread). Jesus revealed that He is the Word of God, the second Person of the Trinity, three divine Persons in one God. He revealed how mankind (not just the Jews, but all people) would be saved, and what they must do to receive this salvation. He brought salvation to the world by allowing Himself to be tortured and killed upon the cross (the death of a criminal). After three days He arose from the dead and taught His disciples another 40 days before He ascended into heaven.
Jesus also founded His Church upon the earth , which is both His Body (the means by which He acts on the earth) and His Bride. This Church was first led by Peter, the Bishop of Rome, and is now led by is successors. He also gave us a means to partake of the saving grace of His sacrifice by giving us the Eucharist, His very own body, blood, soul and divinity, made present every day, throughout the world, in the Holy Sacrifice of the Mass.
While God had revealed to the Jews His name and identity (“I Am Who Am”), Jesus revealed the triune nature of God, the fact that God is Love, and our relationship with God, which is that we are to be the “adopted” children of God, and that God is our loving Father, and that our destiny, if we accept His gift of salvation, is to spend eternity in perfect joy with God, partaking of the very nature of God (we abiding in Him, and He abiding in us).
Well, that’s about as short as I can make it. I don’t think I’ve told any heresies in this telling. If I’ve left anything important out, I’m sure others will catch my omissions – hey, it’s early in the morning.
