John saw it. John watched it. John studied it. John sacrificed for it. John cared for it. John defended it. John fought for it. John taught it. John loved it.
Here's how it's defined.
noun ( pl. truths |troōðz; troōθs|)
the quality or state of being true
• (also the truth) that which is true or in accordance with fact or reality
• a fact or belief that is accepted as true
John was one of Jesus' original 12 disciples...probably the "disciple whom Jesus loved." (John 21:20). John gave and devoted his life to following Jesus. He watched Him heal the sick, hurting, dying, and dead. He saw Jesus display His power over nature by walking on water and calming storms. He listened to Jesus teach incredible truths to the most intelligent men. He was one of the few disciples to watch His beloved Savior be transfigured into His glory and suffer as He prayed in the garden the night of His betrayal. He followed and listened to Jesus' accusers and was there on Golgotha with Mary to watch Jesus die. He was one of the first to witness the empty tomb, and also witnessed the resurrected Jesus numerous times before Jesus ascended to heaven. He watched and heard the commands of Jesus impact and change the lives of thousands of people, and he dedicated the rest of his life to teaching the truth that Jesus taught.
He heard Jesus claim to be the truth (John 14:6), and also listened to Jesus proclaim this:
John 8:31-32, "Jesus said to the people who believed in him, “You are truly my disciples if you remain faithful to my teachings. And you will know the truth, and the truth will set you free.”
Bottom line...John cared about the truth because Jesus cared about the truth. All through 1 John, John battles and argues strongly for the church to hold on to the teachings and example of Jesus (the truth) instead of settling for the ways that seem easy or right to the world. John spends 5 chapters challenging the church to embrace, protect, and proclaim the truth of Jesus. He finishes his book with one simple phrase:
"Dear children, keep away from anything that might take God’s place in your hearts." 1 John 5:21
Battle for the truth in your lives. Let is change you. Protect it. Proclaim it. Don't settle for anything else that tries to squeeze it's way into your life. Stay strong.