John 8:51

‘In very truth I tell you, if a man keeps my word he will never see death'.

The fact that they should recognise was that His words offered life. Those who fully responded to them would never die. Jesus was of course speaking about eternal death. The way to eternal life, He was telling them, was by studying Jesus' words, receiving the truth about Him, believing in Him and responding to Him, and then obeying His teaching. The Pharisees taught that eternal life was obtainable by a constant study of the words of Moses, and a determined effort to obey them as they were expounded by the Rabbis, demonstrating their participation in the God's covenant. Jesus was now replacing Moses and putting Himself in his place.

The Judaisers, probably mainly Pharisees, either could not understand, or probably preferred not to understand. They preferred to take His words literally.