31 Many perhaps, while abandoning the teaching of Christ, were not fully conscious of being led astray by a mirage of glittering phrases, which proclaimed such estrangement as an escape from the slavery in which they were before held; nor did they then foresee the bitter consequences of bartering the truth that sets free, for error which enslaves. They did not realize that, in renouncing the infinitely wise and paternal laws of God, and the unifying and elevating doctrines of Christ's love, they were resigning themselves to the whim of a poor, fickle human wisdom; they spoke of progress, when they were going back; of being raised, when they groveled; of arriving at man's estate, when they stooped to servility. They did not perceive the inability of all human effort to replace the law of Christ by anything equal to it; "they became vain in their thoughts" ( Romans i. 21).
Source: Summi Pontificatus (Vatican.va)