This evening, our small group delved into John 5. We spent a good while pondering Jesus' statement in Jn 5:17, and why it elicits the response of Jn 5:18. In his useful little tome on John's gospel, John Pryor does well to elucidate the inner logic of these verses;
The dialogues of vv. 10-17 are typical of the evangelist's style, and they drive the story on to the point where the evangelist is able to give his summary (in v. 18) of the essential charge of the Jews against Jesus. In the process, Jesus gives his justification for his Sabbath action: just as the Father does not cease to act on the Sabbath, so he acts. Behind this is the rabbinic awareness that since people are born and die on the Sabbath, God cannot be said to be idle on any day, for the gift of life and the work of judgment are divine prerogatives. From his aligning of his activity with that of God, the Jews (= religious authorities) draw two conclusions: Jesus claims equality with God; and this is a self-made claim.[John: Evangelist of the Covenant People - The Narrative & Themes of the Fourth Gospel (Downers Grove, Ill: IVP, 1992); 26-27].
According to the rabbinic tradition (which the religious establishment would no doubt have been familiar with), God does not rest on the Sabbath, since on it he both gives and takes away life. Concordantly, Jesus does not refrain from asserting divine prerogatives on the Sabbath. Just as the Father gives life and death on the day of rest, so the Son gives eschatological life and exerts judgment on the day (cf. Jn 5:24-29). God's life-giving/judgment-exerting activity is now displayed in Christ, so that one must submit to the Son if he/she wishes to honor the Father.