Because of the unbelief the Judeans had when hearing tough topics (and subverted expectations), many of the Judeans followed Jesus no more.
Meanwhile, the original twelve disciples stayed Jesus. With the massive departure going on, Jesus asked the disciples if they too also want to leave (6:67). The quote suggests that Jesus does not "use the force" in making people stay, even though his words have importance.
But in response to the question, Simon Peter saw how the twelve (as a whole) know that Jesus has the words of eternal life (6:68). Peter also proclaimed that Jesus was the Christ and the Son of God (6:69). Thus, Peter and the disciples were willing to trust in Jesus (even though many people are not sticking with him) because of the reliability of the content that Jesus speaks.
Even today, we can still believe in Jesus and even be a disciple of Jesus in our day to day life in spite of other people not doing so because of the reliability of the content that Jesus proclaimed.