The Middle Stoa, which flourished in the 2nd and early 1st centuries bce, was dominated chiefly by two philosophers of Rhodes: Panaetius, its founder, and his disciple Poseidonius. Panaetius organized a Stoic school in Rome before returning to Athens, and Poseidonius was largely responsible for an emphasis on the religious features of the doctrine. Both were antagonistic to the ethical doctrines of Chrysippus, who, they believed, had strayed too far from the Platonic and Aristotelian roots of Stoicism. It may have been because of the considerable time that Panaetius and Poseidonius lived in Rome that the Stoa there turned so much of its emphasis to the moral and religious themes within the Stoic doctrine. Panaetius was highly regarded by Cicero, who used him as a model for his own work. Poseidonius, who had been a disciple of Panaetius in Athens, taught Cicero at his school at Rhodes and later went to Rome and remained there for a time with Cicero. If Poseidonius admired Plato and Aristotle, he was particularly interested—unlike most of his school—in the study of natural and providential phenomena. In presenting the Stoic system in the second book of De natura deorum (45 bce; On the Nature of the Gods), Cicero most probably followed Poseidonius. Because his master, Panaetius, was chiefly concerned with concepts of duty and obligation, it was his studies that served as a model for the De officiis (44 bce; On Duties) of Cicero. Hecaton, another of Panaetius’s students and an active Stoic philosopher, also stressed similar ethical themes.

If Chrysippus is to be commended for his diligence in defending Stoic logic and epistemology against the Skepticism of the New Academy (3rd–2nd century bce), it was chiefly Panaetius and Poseidonius who were responsible for the widespread popularity of Stoicism in Rome. It was precisely their turning of doctrine to themes in moral philosophy and natural science that appealed to the intensely practical Romans. The times perhaps demanded such interests, and with them Stoicism was to become predominantly a philosophy for the individual, showing how—given the vicissitudes of life—one might be stoical. Law, world citizenship, nature, and the benevolent workings of providence and the divine reason were the principal areas of interest of Stoicism at this time.

Such tendencies toward practicality are also well illustrated in the later period of the school (in the first two centuries ce) in the writings of Lucius Seneca, a Roman statesman; of Epictetus, a former slave; and of Marcus Aurelius, a Roman emperor. Both style and content in Seneca’s Libri morales (Moral Essays) and Epistulae morales (Moral Letters) reinforce the new direction in Stoic thought. The Encheiridion (Manual) of Epictetus and the Meditations of Marcus Aurelius furthered the sublime and yet personal consolation of the Stoic message and increasingly showed the strength of its rivalry to the burgeoning power of the new Christianity. The mark of a guide, of the religious teacher, is preeminent in these writings. It is difficult to establish with any precision, however, the extent of Stoic influence by the time of the first half of the 2nd century ce. So popular had these ideas become that many specifically Stoic terms (viz., right reason, comprehension, assent, indifference, logos, and natural law), as well as the notion of the wise person, were commonly used in debate and intellectual disputes.

Stoic elements in Pauline and patristic thought

There is much disagreement as to the measure of Stoic influence on the writings of St. Paul the Apostle. At Tarsus, Paul certainly had opportunities for hearing Stoic lectures on philosophy. And it may be that his discussion of nature and the teaching of it (1 Corinthians 11:14) is Stoic in origin, for it has a parallel in the Manual of Epictetus (1.16, 10). Although not a Stoic technical term, syneidēsis, which Paul used as “conscience,” was generally employed by Stoic philosophers. In 1 Corinthians 13 and in the report of Paul’s speech at Athens (Acts 17), there is much that is Hellenistic, more than a little tinged by Stoic elements—e.g., the arguments concerning the natural belief in God and the belief that human existence is in God.

The assimilation of Stoic elements by the Church Fathers was generally better understood by the 4th century. Stoic influence can be seen, for example, in the relation between reason and the passions in the works of St. Ambrose, one of the great scholars of the church, and of Marcus Minucius Felix, a Christian Apologist. Each took a wealth of ideas from Stoic morality as Cicero had interpreted it in De officiis. In general, whereas the emerging Christian morality affirmed its originality, it also assimilated much of the pagan literature, the more congenial elements of which were essentially Stoic.

Earlier, in the 3rd century, Quintus Tertullian, often called the father of Latin Christian literature, seems to have been versed in Stoic philosophy—e.g., in his theory of the agreement between the supernatural and the human soul, in his use of the Stoic tenet that from a truth there follow truths, and in his employment of the idea of universal consent. Even in his polemical writings, which reveal an unrelenting hostility to pagan philosophy, Tertullian showed a fundamental grasp and appreciation of such Stoic themes as the world logos and the relation of body to soul. This is well illustrated in his argument against the Stoics, particularly on their theme that God is a corporeal being and identified with reason as inherent in matter—also to be found in his polemics against Marcion, father of a heretical Christian sect (the Marcionites), and against Hermogenes of Tarsus, author of an important digest of rhetoric. Yet, in his doctrine of the Word, he appealed directly to Zeno and Cleanthes. Another important polemic against the Stoics is found in the treatise Contra Celsum, by Origen, the most influential Greek theologian of the 3rd century, in which he argued at some length against Stoic doctrines linking God to matter.

Also, St. Cyprian, bishop of Carthage in the 3rd century, revealed the currency of Stoic views—e.g., in his Ad Demetrianum (To Demetrius), a denunciation of an enemy to Christianity, in which Cyprian castigates the ill treatment of slaves (who, no less than their masters, are formed of the same matter and endowed with the same soul and live according to the same law). The beliefs in human brotherhood and in the world as a great city, commonly found in early Christian literature, were current Stoic themes. The Christian attitude appears in what St. Paul said of baptism: “You are all sons of God through Faith. For as many of you as were baptized into Christ have put on Christ” (Galatians 3:26–27).