What kind of thing is the faith by which we are justified?
It is an invisible faith, not a work you perform or strive at. Justification is God’s accomplished act through Christ’s death, burial, and resurrection, in which we had nothing of our own energy. It is not a work of the flesh, but the work of God, so it is not earned by human effort.
Les explains that the faith by which we are justified is something invisible, something you don’t work at. He contrasts this with those who claim to be Christians but cannot accept such security, insisting you must work, strive, and be careful or you will lose it. They end up in perpetual insecurity because no one has the wherewithal to hang on by their own strength. The context wraps up everything learned in the previous seven chapters into Romans 8: there is therefore now no condemnation because we have been justified by faith, because Christ has died, been buried, and risen from the dead, all accomplished as an act of God in which we had nothing of our own energy. Nothing in that was done in the flesh; it was all the work of God.