Pope Francis draws very widely from the testimony of the saints in his documents. In this paragraph, continuing commentary on the unintentional cluttering of the preaching of the Gospel.
59. Once we believe that everything depends on human effort as channeled by ecclesial rules and structures, we unconsciously complicate the Gospel and become enslaved to a blueprint that leaves few openings for the working of grace. Saint Thomas Aquinas reminded us that the precepts added to the Gospel by the Church should be imposed with moderation “lest the conduct of the faithful become burdensome”, for then our religion would become a form of servitude. [Summa Theologiae I-II, q. 107, art. 4]
The Holy Father doesn’t give an example here, but we might guess. Church rules and structures can complement the Gospel message and guide people well. They can also obscure the priorities as Jesus may have offered, most notably on aspects such as mercy.