After hearing Jesus in the Capernaum synagogue, many disciples respond, 'This is an hard saying; who can hear it?' Jesus asks whether his words offend them and points toward the Son of Man ascending where he was before.
The result is an actual loss of followers.
The Greek adjective skleros can describe something hard, harsh, severe, or difficult to accept, while the verb translated 'offend' carries the idea of stumbling. The issue is therefore more than failure to understand vocabulary. Jesus' claims about heavenly origin, flesh, blood, and life provoke resistance.
His reference to ascending 'where he was before' returns the dispute to the preexistence and heavenly-origin claims running throughout the chapter.
The language explains why the narrative does not end with everyone receiving a clearer explanation and remaining. Some hearers understand enough of the claim to reject it.
John presents discipleship as capable of reaching a crisis point when Jesus refuses to fit expectations. The Twelve are then confronted with the same choice rather than protected from it.