25 This same charity we desire that all should earnestly endeavor to attain, taking special occasion from the extraordinary feasts in honour of the Immaculate Conception of the Blessed Virgin. Oh how bitterly and fiercely is Jesus Christ now being persecuted, and the most holy religion which he founded! And how grave is the peril that threatens many of being drawn away by the errors that are afoot on all sides, to the abandonment of the faith! "Then let him who thinks he stands take heed lest he fall" (I Cor . x., 12). And let all, with humble prayer and entreaty, implore of God, through the intercession of Mary, that those who have abandoned the truth may repent. We know, indeed, from experience that such prayer, born of charity and relying on the Virgin, has never been vain. True, even in the future the strife against the Church will never cease, "for there must be also heresies, that they also who are reproved may be made manifest among you" (I Cor . xi., 19). But neither will the Virgin ever cease to succor us in our trials, however grave they be, and to carry on the fight fought by her since her conception, so that every day we may repeat: "To-day the head of the serpent of old was crushed by her" (Office Immac. Con., 11. Vespers, Magnif. ).
Source: Ad Diem Illum Laetissimum (Vatican.va)