Jesus said this to indicate the kind of death by which Peter would glorify God. And after He had said this, He told him, “Follow Me.”
Textus Receptus (TR) - John 21:19 >Τοῦτο δὲ εἶπε, σημαίνων ποίῳ θανάτῳ δοξάσει τὸν Θεόν. Καὶ τοῦτο εἰπὼν λέγει αὐτῷ, Ἀκολούθει μοι.