"The Jews, therefore, murmured at Him, because He had said: I am the living bread which came down from heaven." (John vi. 41.) "This murmuring at the doctrine of our Lord Jesus Christ is," says St. Cyrillus, "the inheritance which was bequeathed to the Jews by their forefathers, who lived at the time of Moses."Would to God that this inheritance had been transmitted to the Jews only; but, alas! there is no class of men which is free from such murmurers. Our Lord's doctrine is murmured at by infidels when they hear Him say: "He that believeth not shall be condemned" (Mark xvi. 16) ..."because he believeth not in the name of the only begotten Son of God." (John iii. 18.)The doctrine of our Lord is murmured at by Protestants, when He declares: "Not every one that saith to Me, Lord, Lord, shall enter into the kingdom of heaven, but he that doth the will of My Father who is in Heaven, he shall enter into the kingdom of heaven." (Matt. vii. 21.) The will of God has not been taught by Luther, or Calvin, or Henry VIII, or John Wesley, or by another man who invented certain doctrines, and founded a sect according to his own private notions, but it has been taught by Me, the Son of God, Who have charged Peter and his lawful successors to do the same. Upon him I have built My Church; to him and his lawful successors I have said: "He who heareth you heareth Me, and he that despiseth you despiseth Me, and he who despiseth Me despiseth Him that sent Me." One who does not do this will be condemned. "There is a way [the Protestant religion] that seemeth to a man right, and the ends therefore lead to death." (Prov. xvi. 25.)Sinners murmur when our Blessed Savior preaches: "I say to you that unless you shall do penance, you shall all likewise perish." (Luke xiii. 3.) The rich also complain, when He threatens "Woe to you that are rich, for you have your consolation." (Luke vi. 24.) The poor are dissatisfied when He teaches: "Blessed are the poor in spirit." (Math, v. 3.) The learned reject His doctrine when he warns: "Amen I say to you: unless you be converted and become as little children, you shall not enter into the kingdom of heaven." (Math. xviii. 3.) The young are displeased when He exclaims, "Woe to you that now laugh, for you shall mourn and weep." (Luke vi. 25.) Those who are tempted or afflicted, murmur when He exhorts them by His words and example: "Not my will but Thine be done." (Luke xxii. 42.) The lukewarm are displeased when He tells them: "Because thou art lukewarm, and neither cold nor hot, I will begin to vomit thee out of My mouth." (Apoc. iii. 16.) Finally, the greater part of men murmur at our Lord, when He teaches: "The kingdom of heaven suffereth violence and the violent bear it away." (Matt. xi. 12.) They complain with the unfaithful disciples of our Lord, "these are hard sayings who can hear them." (John vi. 61.)There are still many, it is true, who will say with St. Peter and the other Apostles: "Lord, to whom shall we go? Thou hast the words of eternal life, and we have believed and have known that Thou art Christ the Son of God." (John vi. 69, 70.) But how many, even among these, will murmur, not indeed at Christ's doctrine, but at heretics, unbelievers and great sinners? How many are there who, like the Apostles, not knowing of what spirit they are, wish that fire should come down from heaven to consume them (Luke ix. 54, 55), for not believing, in spite of so many miracles and evident proofs, confirming the truth of the Catholic religion? To all these our Lord answers with divine sweetness: "Murmur not among yourselves: no man can come to Me, except the Father, who hath sent Me, draw him." (John vi. 44.) As to all those of you, He means to say, who believe in Me and live up to My doctrine, you ought not to murmur at infidels, heretics or nominal Christians, on account of their infidelity, false belief or bad life, but you should remember that faith, especially practical faith, is a supernatural gift of God, and that no one can have true faith in Him unless it is granted by My heavenly Father. Since they are not as yet drawn by the Father, you should not feel indignant or treat them with severity, but rather pray to the Father that He may draw them sweetly, but powerfully, by enlightening their understanding to know the true faith, and by exciting their will to embrace it in practice, and thus they will be united with you in the same religion.
But as to you who do not believe My doctrine, or believe only a part of it, or live not according to it, neither ought you to murmur at Me and My doctrine or at those who believe truly in Me, because My Father has drawn them. Pray you, too, to My Father that He may draw you also, by removing from your understanding the darkness which prevents you from knowing My Church and the truths she teaches. Pray that He may remove from your heart the coldness and indifference which prevents you from loving the truth, and from your will the reluctance and resistance which prevents you from embracing it. For this purpose, you should often say to God in all sincerity: "Our Father, who art in heaven, if there are still more truths which I must know and practise, in order to be saved, I beseech Thee, for the sake of Jesus Christ, permit me to know them in whatever way it pleaseth Thee to manifest them to me. Give me a good will that I may embrace them and practise faithfully what they command, until the end of my life." If you pray perseveringly, in this manner, rest assured that you also will be drawn by My Father, to live and die with My true followers in the same faith. All your unjust murmurs and complaints would soon be changed into joy, as I have promised when I said: "Ask and you shall receive, that your joy may be full," (John xvi. 24), for My Father "is rich unto all that call upon Him," (Rom. x. 12) in My name, for the sake of which I will grant that life of which I have said: "I am come that they may have life, and have it more abundantly," (John x. 10), here by My exuberant grace and hereafter by My unspeakable glory.This doctrine, of such vital importance for the salvation of mankind, is too seldom preached, little understood, and still less put in practice," God thus permitting it," says St. Alphonsus, "in punishment for the sins of men.""And now, brethren, as you are the ancients among the people of God, and their very soul resteth upon you, comfort their hearts by your speech" (Judith viii. 21), by explaining to them, as often and as plainly as possible, the great necessity of this doctrine on prayer, as well as the right manner of practising it, in order to derive therefrom all possible advantage.... "Ah, my dear brethren," wrote Pope Celestine to the Bishops of France, "let prayer never leave your hearts, and the grace and mercy of God will never leave your souls. Rest assured that the Lord will never withdraw from you, nor cease to enlighten, guide and protect you as long as you pray to Him. You complain of the difficulty of saving your souls in the midst of a corrupt world, in which you are exposed to so many dangers. Do you wish to escape them all and to fear none? Arm yourselves with prayer. Prayer was the daily food and strength of the prophet; it was his whole delight; he understood but too well all its advantages."
This is the preface to a profound yet simple work on prayer by Redemptorist Father Michael Müller, which we thoroughly recommend. It is available here on Google Books and on the Internet Archive: Prayer, the Key to Salvation