In the first volume on the Sacraments, St. Robert follows the order of teaching in the Council of Trent to defend Catholic truth against the teaching of early Protestants. In his work On the Sacraments in General, He defends the Catholic notion of Sacraments as well as their use. He looks to the definition of a Sacrament, and then the matter and form, the author of the Sacraments, the difference between the Sacraments used in the Old Testament and in the New, whether one can change the matter and form, as well as the notion of the minister of the Sacrament, and what he must intend to do for the Sacraments to be valid. According St. Robert's custom, he defends all of his argumentation first from Scripture, and then from Tradition, providing penetrating exegetical commentary combined with the consensus of the teaching of the Fathers in defense of the universal Catholic teaching upheld at Trent. He also adds argumentation from reason, and takes head on the arguments of the early Protestants at every turn, showing the contradictions of Luther, the misuse of Scripture of Calvin, and the foolishness of Martin Chemnitz.

In book II, St. Robert defends their effects and their number, as well as the validity of ceremonies approved by the Church. St. Robert demonstrates, on the sure foundation of Scripture and the Fathers, that the Sacraments obtain their effect ex opere operato, what is meant by that term and how the Protestants misunderstood it. He also defends the Scholastics against attacks and abuses which Calvin and Chemnitz heap on them. Moving on to other matters, St. Robert defends that the effect of grace in the Sacraments is only found ex opere operato in the New Testament. Lastly, he proves that the Church has the power to effect new ceremonies for the Sacraments, and defends the use of the Latin language in the Western Church, or Greek and Aramaic in the Eastern Churches.

In On Baptism and Confirmation, he defends the Catholic teaching on regeneration in Baptism, along with the existence and effects of Confirmation. As always, he draws his arguments from Scripture, and the Fathers, combining faithful exegesis with the witness of the Church in all ages. St. Robert defends the form and matter of Baptism, catches the Protestants in contradictions, and defends that it is necessary for salvation, whereas faith alone does not justify. He goes on to show that it can be administered to children against the Anabaptists, and the difference between the Baptism of Christ and that of John. Then, on Confirmation, St. Robert shows from Scripture and Tradition that Confirmation is attested to by the actions of the Apostles, and the teaching which they passed down to the Fathers, along with its effects, ceremonies, and refutation of Protestant arguments against it. Apart from Calvin, Bellarmine makes a systematic refutation of Martin Chemnitz's Examination of the Council of Trent on any subjects touching upon the Sacraments.

"For five years in Chablais, I preached with no books other than the Bible, and the Great Bellarmine."

-St. Francis de Sales