The thrust of this chapter is that whatever God ordains or permits to happen to us, it is for the best. Consolation or desolation, rejoicing or suffering, it is all to be accepted willingly, even joyfully. Only sin can harm us and move us away from God — it alone we ask to be preserved from.
This day we get the conclusion of the Sermon on the Mount. After Jesus has provided this extended teaching (covering three chapters), He emphasizes the absolute necessity of listening to (that is, not just hearing, but processing) all He says and acting on it. This is the solid foundation of faith and discipleship. Simply calling out the name of the Lord while in turn ignoring or defying His teachings risks the danger of eternal damnation.
The first line of today’s Gospel, found in the headline, is one of my “go to” verses when defending the Catholic Faith against those who believe in “once saved, always saved.” Jesus spent His entire earthly ministry instructing His followers on appropriate actions and often warning of the dire consequences of not doing so; it is incredible that anyone could possibly think that a simple one-time affirmation of Jesus as Lord and Savior is sufficient without a lifestyle that corresponds to His directives. So, like the disciple in Kempis, we individually are to desire that “my will remains right and firm in You.” Through prayer, study (especially of Scripture and the Catechism), and spiritual direction (if possible), we strive to know God’s will and faithfully follow it with the help of the Sacraments.