Are we guaranteed to go to heaven simply by praying the Rosary faithfully each day?
Depending on which quotations (prooftexts?) you read from the saints, you might be led to think that. Many publications quote such things precisely to encourage the frequent prayer of the Holy Rosary. And indeed, I think it is correct to say that if you pray the Rosary with a sincere heart, as part of a heartfelt life of devotion and loving service of God, it will certainly obtain for you many special graces, among which may be that of final perseverance. But be on guard against anyone who offers the daily Rosary as a talisman or magic formula!
While researching this topic, I came across a chapter of True Devotion to Mary that is very relevant to the question at hand:
97. Presumptuous devotees are sinners who give full rein to their passions or their love of the world, and who, under the fair name of Christian and servant of our Lady, conceal pride, avarice, lust, drunkenness, anger, swearing, slandering, injustice and other vices. They sleep peacefully in their wicked habits, without making any great effort to correct them, believing that their devotion to our Lady gives them this sort of liberty. They convince themselves that God will forgive them, that they will not die without confession, that they will not be lost for all eternity. They take all this for granted because they say the Rosary, fast on Saturdays, are enrolled in the Confraternity of the Holy Rosary or the Scapular, or a sodality of our Lady, wear the medal or the little chain of our Lady.
When you tell them that such a devotion is only an illusion of the devil and a dangerous presumption which may well ruin them, they refuse to believe you. God is good and merciful, they reply, and he has not made us to damn us. No man is without sin. We will not die without confession, and a good act of contrition at death is all that is needed. Moreover, they say they have devotion to our Lady; that they wear the scapular; that they recite faithfully and humbly every day the seven Our Fathers and seven Hail Marys in her honour; that sometimes they even say the Rosary and the Office of our Lady, as well as fasting and performing other good works.
Blinding themselves still more, they quote stories they have heard or read – whether true or false does not bother them – which relate how people who had died in mortal sin were brought back to life again to go to confession, or how their soul was miraculously retained in their bodies until confession, because in their lifetime they said a few prayers or performed a few pious acts, in honour of our Lady. Others are supposed to have obtained from God at the moment of death, through the merciful intercession of the Blessed Virgin, sorrow and pardon for their sins, and so were saved. Accordingly, these people expect the same thing to happen to them.
98. Nothing in our Christian religion is so deserving of condemnation as this diabolical presumption. How can we truthfully claim to love and honour the Blessed Virgin when by our sins we pitilessly wound, pierce, crucify and outrage her Son? If Mary made it a rule to save by her mercy this sort of person, she would be condoning wickedness and helping to outrage and crucify her Son. Who would even dare to think of such a thing?
99. I declare that such an abuse of devotion to her is a horrible sacrilege and, next to an unworthy Communion, is the greatest and the least pardonable sin, because devotion to our Lady is the holiest and best after devotion to the Blessed Sacrament.
I admit that to be truly devoted to our Lady, it is not absolutely necessary to be so holy as to avoid all sin, although this is desirable. But at least it is necessary (note what I am going to say), (1) to be genuinely determined to avoid at least all mortal sin, which outrages the Mother as well as the Son; (2) to practise self-restraint in order to avoid sin; (3) to join her confraternities, say the Rosary and other prayers, fast on Saturdays, and so on.
100. Such means are surprisingly effective in converting even the hardened sinner. Should you be such a sinner, with one foot in the abyss, I advise you to do as I have said. But there is an essential condition. You must perform these good works solely to obtain from God, through the intercession of our Lady, the grace to regret your sins, obtain pardon for them and overcome your evil habits, and not to live complacently in the state of sin, disregarding the warning voice of conscience, the example of our Lord and the saints, and the teaching of the holy gospel. (Source)
The daily Rosary? Yes! But as true devotees – not as presumptuous servants!