One is only able to really appreciate the paradox Kempis illustrates (“troublesome and grievous” temptation should lead to one being “humbled, purified, and instructed”) when that person successfully overcomes that temptation (1,13). Isaiah’s plea in today’s first reading (Is 58:7-10) to remove from oneself “false accusation and malicious speech.” The reward for not giving into the temptation to do such: light rises, gloom fades.
Sin is gloom and darkness, holiness is light. God is light. Jesus is “the light of the world” (John 8:12). Jesus is “the way and the truth and the life” (Jn 14:6). When we overcome temptation and stop our habitual sin, we are open to be “humbled, purified, and instructed.” We humble ourselves before God and others (no “false accusation” or “malicious speech”), we are purified through grace for the forgiveness of our sins, and we are instructed in the truth of the Gospel by being more open to it — the light exposes the dust and dirt on our souls and provides us the means to tidy up, thus removing obstacles to achieving holiness. Through perseverance, in the end, and forever, we will enjoy this reality:
The city had no need of sun or moon to shine on it, for the glory of God gave it light, and its lamp was the Lamb. (Rev 21:23)