As we go through the Gospel, things get very real. The Gospel is not a story but is given to us so as to bring us into it, so as to change us.
Jesus suffers and speaks of our share in that. What is our suffering? Whatever it takes for us to accept Jesus’ act of love, so that we are animated by Him (rather than by fallen human nature.) This 'suffering' saves!
Our suffering takes the form, not of crucifixion, but of humiliation, of rejecting our pride, of changing our life, of going against the culture.
But if we really embrace Jesus' act of love, nothing can get in our way.
Note in the Gospel that when Peter answers Jesus' question, he gives the right answer. But he has not learned yet what that means for him. This is why Jesus tells him not to tell anyone about him; Peter is not yet ready to give an authentic witness or understanding. Jesus knows that Peter still has to change - to be transformed by grace. In grace he will give an authentic witness.