Truly, truly, I say to you, whoever hears my word and believes Him who sent me has eternal life. He does not come into judgment, but has passed from death to life.

Background
John was writing to mostly second generation Christians around 85-90 AD.  They needed reinforcement on key things Jesus did and taught because Gnostics, Orthodox Jews and other cultural groups would have been sowing seeds of doubt.

John wanted to make clear statements about these key things.  In this section of his letter, he provides strong validation of Jesus as the Son of God and that He rose from the dead.

John made it perfectly clear that faith in Jesus was complete redemption from sin which was not offered by animal sacrifices.

What it meant to the original audience
With people trying to pull them back into needing to earn God’s love (“works” based salvation), this was likely very comforting to them.

For us today
Today, we still tend to want to earn favor with God and go to Heaven by “being a basically good person”.  God, like a loving and wise parent, wants us to understand we are loved even when we mess up badly.

God made His love clear:  We can 100% KNOW that we are forgiven of all our sins past, present and future if we acknowledge we are sinners and accept Jesus’ death on the cross as the consequences payment for all our sins.