What is the most important thing I should know?

The Bible tells us that Jesus Christ paid the penalty for all of men's sins, allowing us to have a relationship with God. Jesus’ death on the cross makes it possible for us to be released from the bondage of sin, and to live in heaven with God for eternity. Faith in Christ is a requirement of salvation (Ephesians 2:8-9).

God created mankind (Genesis 1:26-27)
God Himself is holy (1 John 1:5)
God requires holiness of mankind (Galatians 3:10)
God gave mankind the opportunity to choose whether to obey God's laws. Adam, the first man, chose to disobey God's law, and that moment represents the fall of mankind (Romans 5:12).
Everyone has sinned (Romans 3:23)
The penalty for our sin is death--not just physical death, but spiritual death in the form of eternal separation from God (Romans 6:23)
Our best efforts to live up to God's standard are not good enough (Isaiah 64:6). We cannot erase our own sin.
Jesus Christ came to earth as God in human form (Colossians 2:9)
Christ died for those who have sinned (I Peter 3:18, Romans 5:6-8)
Christ rose from the dead and is alive today (Romans 6:8-11, I Corinthians 15:4)
To be saved, one must believe on Jesus and declare Him Lord (Romans 10:9)