You “WILL know” the truth He said. Jesus was talking to those who believed in him. Yet they were not free for they did not yet "know the truth". The truth would come through the Apostle Paul.
Believing in Jesus does not set you free, only believing what Jesus has done for humanity sets us free.
So, what was the truth that would "set them free"?
"The wages of sin is death, but the free gift of God is eternal life through Jesus Christ our Lord." (Romans 6:23)
“…by His death He (Jesus) might break the power of him who holds the power of death—that is, the devil—and free those who all their lives were held in slavery by their FEAR of death." (Hebrews 2:14-16)
Humanity is “held in slavery by their fear of death”. We fear death because we do not believe that humanity is to be saved out of it. When we “hold to” the expectation of our resurrection we have the faith like Abraham, the father of all who have faith...
“…because the law brings wrath…the promise comes by faith, so that it may be by grace and may be guaranteed to all Abraham's offspring--not only to those who are of the law but also to those who have the faith of Abraham. He is the father of us all. He is our father in the sight of God, in whom he believed--the God who gives life to the dead and calls into being things that were not.” (Romans 4:15-17)
After God gives "life to the dead" every human will know the truth and set free from the fear of death. For now, only those who have heard and believe in the resurrection enjoy the free gift of eternal life - we have been set free from the fear of death.
“Now the just shall live by faith..." (Hebrews 10:38)
As in Adam all die, even so in Christ shall all be made alive." (1 Corinthians 15:22)
"No longer will they teach their neighbor, or say to one another, ‘Know the Lord,’ because they will all know me, from the least of them to the greatest. For I will forgive their wickedness and will remember their sins no more.” (Hebrews 8:11-12)
There is a lot of wickedness in this world. We should fight to eliminate that wickedness - but after the wicked get the death coming to them, God intends to "forgive their wickedness and remember their sins no more". That is what a loving, merciful, and gracious God does for humanity through His Son Jesus.
There is a lot of wickedness in this world. We should fight to eliminate that wickedness - but after the wicked get the death coming to them, God intends to "forgive their wickedness and remember their sins no more". That is what a loving, merciful, and gracious God does for humanity through His Son Jesus.