Might be like Sunday church service, but here it goes.
Ok the basic belief of Christianity, is that there is one God. That one God created everything to ever exist, and to exist in the future. He created us with free will so we can show him true, undeniable love. And in turn, he gave us a beautiful world to live in, vast amounts of vegetation, and things to discover and explore.
Everything was perfect until Adam, and Eve (the first two humans) sinned against God. Once Adam and Eve sinned, the perfect bond between man and God was destroyed, and the curse of sin from the devil was released upon the world, because of Adam and Eve. God wants to be with us, but can't because he is perfect, and we are not. So to bridge the gap, and give us a way to be with him, in his presence, he sent himself/son, down to earth in the form of a human.
His name was Jesus, Christ the Lord. He lived a perfect sinless life as a human, and proved to everyone, and himself that it was possible to be sinless as a human, even with the curse of sin bestowed upon humanity. Then Jesus died for our sins, using his perfect life to cleanse all of our sins. After he died, he rose again three days later, and was returned to Heaven/paradise.
Now obviously we are still sinners so we cant just go up to heaven, because our hearts are still full of sin. So God gives us a way. The only thing you have to do to get into heaven, is to ask him into your heart, and ask him to help guide you through life in the best way possible, and I don't mean just ask God randomly with no purpose and go on about your life, but to truly believe he exists with faith, and to truly ask him into your heart.
This is the basic belief of Christianity for all of those people out there who think we try to enforce this belief onto others, persecutor, and other things people might say.