The central message of the Bible is that we can know for sure where we will spend eternity.  The Bible teaches us that we are all sinners, and that we are unable to get to heaven on our own.  For all have sinned, and come short of the glory of God (Romans 3:23).  Furthermore, the punishment for sin is death.  For the wages of sin is death (Romans 6:23).

In the midst of this dilemma, we have a way out—Jesus Christ, the only begotten Son of God.  For God so loved the world, that he gave his only begotten Son, that whosoever believeth in him should not perish, but have everlasting life (John 3:16).

We must receive His gift of eternal life.  For whosoever shall call upon the name of the Lord shall be saved (Romans 10:13).  It is by placing our total faith in Christ and His shed blood that we can know for sure we will spend eternity in Heaven.