Charles H. Spurgeon on Psalms 71:17

Wondrous works. Observe that he calls the blessing of divine aid so often received in affliction, wondrous works. By this expression, he shows us, with what grievous perils he was tossed; then how he had been snatched from them by the hand of God, contrary to the expectation of all men. Therefore, God is wonderful among his saints. To this end the adversities of the saints tend, that they may show forth in them the wonderful works of God. Musculus.

Verses 17-18. The integrity of our hearts and ways, in former walkings after God, and service for God, may by faith in Christ, as in all our justification, be pleaded. See also Isa 38:3 and Ps 119:10. The Lord himself maketh it to himself a motive to show mercy to his people (Isa 63:8 Jer 2:2); only we must use this plea more rarely and sparingly, in a self denying way, in faith in Christ's righteousness, as made ours. Thomas Cobbet.