After spending two years at Bourges he returned to Paris, and began a correspondence with Fronton du Duc, the editor of John Chrysostom. In 1605 he became a Jesuit, taught rhetoric at Reims (1609), La Flèche (1613), and at the Collège de Clermont (1618). During this last period he began a correspondence with the Bishop of Orléans, Gabriel de Laubépine (Albaspinaeus), on the first year of the primitive Church. Beginning in 1622, he taught positive theology for twenty-two years, and during this time he left France on only two occasions: first in 1629, to teach ecclesiastical history at Madrid at the invitation of Philip IV; second in 1639 to become a cardinal at Rome where Pope Urban VIII wanted him. At sixty years of age he stopped teaching, but retained his office of librarian, in which he had succeeded Fronton du Duc (1623), and devoted the rest of his life to his great work, the Dogmata theologica. He died in Paris.
The complete list of his works fills twenty-five columns in Sommervogel: he treats of chronology, history, philosophy, polemics, patristics, and the history of dogma. His first edition of the works of Synesius appeared in 1612, undertaken ten years earlier at the advice of Casaubon (Synesii episcopi Cyrenensis opera, new ed., 1633); in 1613 and 1614 the discourses of Themistius and Julian (new ed., 1630); in 1616 the Breviarium historicum Nicephori; then, after some poetical and oratorical works, an edition of Epiphanius in two volumes (1622; new ed., 1632), which had been undertaken at the advice of Jacques Gretser, S.J., and was originally intended only as a revised translation of Janus Cornarius. In 1622 and 1623 appeared the Mastigophores, three pamphlets, and the notes dealing with Saumaise's Tertullian, a bitter polemical work.
Among his previous writings, Pétau had inserted some masterly dissertations on chronology; in 1627 he brought out his De doctrina temporum, and later the Tabulae chronologicae (1628, 1629, 1633, 1657). It surpassed Scaliger's De Emendatione temporum (Paris, 1583), and prepared the ground for the works of the Benedictines. A summary of it appeared in 1633 (1635, 1641, etc.) under the title of Rationarium temporum, of which numerous reprints and translations into French, English, and Italian have been made.
About the same time he wrote poetical works in Greek and in Latin and dissertations (often of a polemical nature) against Grotius, Saumaise, Arnauld, and others. His paraphrase of the Psalms in Greek verse was dedicated in 1637 to Pope Urban VIII. Finally there appeared in 1643 the first three volumes of the Dogmata theologica (dated 1644); the fourth and fifth volumes were published in 1650; the work was incomplete at Pétau's death, and despite several attempts was never continued. Numerous editions of the "Dogmata theologica" have been published, including that by the Calvinist Jean Le Clerc, published in Antwerp in 1700; the last edition was brought out in eight volumes by J. B. Fournials (Paris, 1866-8). In 1757 F. A. Zaccaria, S.J., republished the work in Venice with notes and dissertations; in 1857 Passaglia and Schrader undertook a similar work, but they produced only the first volume. His letters, Epistolarum libri tres, were published after his death; though far from being complete, they give an idea of his close acquaintance with the most famous men in Europe of his time; they also furnish valuable information on the composition of his works and his method.
Petau's claim to fame chiefly rests on his vast, but unfinished, De theologicis dogmatibus, the first systematic attempt ever made to treat the development of Christian doctrine from the historical point of view.
The reputation Pétau enjoyed during his lifetime was especially due to his work on chronology; numerous eulogies were pronounced on him by his contemporaries, including Pierre Daniel Huet, Henri Valois, Hugh Grotius, Isaac Voss, F. Clericus, and Henry Noris. His chronological work has long since been surpassed, and a list of errors — inevitable at the period — could be drawn up even in the case of this man who boasted that he counted no less than eight thousand mistakes in the Annals of Baronius. But the great glory, which in the eyes of posterity surrounds the name of Pétau, is due to his patristic works and his importance in the history of dogma. With good reason he may be styled the "Father of the History of Dogma". The success of his work in this sphere was slow to make itself felt — it brought upon the author accusations even from within his order — but it was highly esteemed by his pupils and far-seeing friends (e.g., Valois and Huet).
He had at hand only very imperfect editions of the Fathers of the Church, all inferior to the later work of the Benedictines. Many of the known texts of their works only existed in translations, or in late and poorly studied manuscripts, and his predecessors in this line were few and practically everything had to be created. What he wanted had already been outlined by Melchior Cano, O.P., in his work De locis theologicis. Here we pass from theory to practice and we find a master at once.
The originality of Pétau's work has been questioned; it may have been inspired, it is said, by a similar treatise of Oregius (Agostino Oreggi, Cardinal, d. 1635), as Zöckler maintains, or by the Confessio catholica of John Gerhard (d. 1627), as conjectured by Eckstein. But the Confessio catholica has a quite different aim, as is stated on the very first page; whole treatises, as for instance that on Christ, have but scanty quotations from three or four Fathers, and present nothing similar to the long historical developments of the sixteen books De Incarnatione Verbi of Pétau.
The relationship with Oregius, which rests solely on a conversation of a religious of the Minims of Dijon related in the Voyage littéraire de deux bénédictins (Paris, 1717, p. 147), was examined in detail by François Oudin, S.J. in the Mémoires de Trévoux (July, 1718, pp. 109-33).
The state of religious strife during the days succeeding the Council of Trent drew all minds towards the primitive ages of the Church concerning which certain ancient documents were being discovered, while the excessive subtlety of many Scholastics of the decadence instigated a return towards positive sources. Pétau was no doubt inspired by the same ideas, but the execution of the work is completely his own.
His aim and purpose are set forth by his dedicatory letter to the General of the Jesuits (Epist., III, liv), and in several parts of his Prolegomena. His method reveals all the resources which the sciences of history and philosophy have furnished to the theologians. He declares his opinion with full liberty for example, concerning the opinion of Augustine of Hippo on the problem of predestination, or the ideas on the Trinity of the ante-Nicene writers.
The work furnished a copious supply of documents; for theologians it has been a store of patristic arguments. Pétau, like Cano, took the greatest pains with his literary style. He exaggerates the faults of Scholasticism; but on the other hand he defends it against the accusations of Erasmus. We still find the controversialist in the author of the Dogmata; after giving the history of each dogma, he adds the refutation of new errors.
In his polemical writings his style was bitter; here and there he is more gentle, as when engaged in discussions with Grotius, who was drawing near the Catholic faith. The memory of Pétau was celebrated the day after his death by Henri Valois, one of his best pupils, and by Leo Allatius in a Greek poem composed at the request of Pope Urban VIII.