Christianity started out as Roman Catholics.   Christians believe there is only one God, but He is three persons, Father, Son and Holy Spirit.  Christians know that Christ  died on the cross to forgive the sins of all believers.  Because of Him, they can receive eternal life in heaven. 
    When God made earth, he made Adam and Eve without sin.   When they ate the forbidden fruit from the Tree of Life, they realized their sin and now  all are  born sinners.  Christians believe sin is anything against God's law, the Ten Commandments.  If you do sin, you need to pray to God and He will forgive you if you believe in Jesus Christ as your Lord and Savior.