Augustine was easily the best - if not also the very first - psychologist in the ancient world. He is an essential source for both contemporary depth psychology and existentialist philosophy.
The observations and descriptions of Augustine about human motives and emotions, his depth analyses of will and thought in their interaction, and his exploration of the inner nature of the human self have established one of the main traditions in European conceptions of human nature.
And this remains true even to our own era, which is a great achievement for somebody who lived as long ago as the fifth century. His descriptions of subjective events (e.g., the will, experienced freedom of the will, the self) began the tradition of inner thought (introspection) and phenomenology in psychology.
Augustine discussed the perception of time and concluded that time is an inner experience, i.e., it is psychological. His psychological ideas were first published in Confessions, one of the earliest great revelations of self. By writing this unique book during the three years after he became Bishop of Hippo, Augustine took a spiritual and psychological assessment of himself and his past thoughts and behaviour. In that book, Augustine explored the psychology of conversion and expressed the conviction that God had transformed his soul. He described his spiritual unease (restlessness) as an example of that inner movement towards God that he believed as characterizing human life. What Augustine there described as a motion towards unity or wholeness, psychology today terms as "congruency" (i.e., various factors coming together in harmony).
In the Confessions Augustine addresses such consequential issues as family relations, memory, conversion, mystical realities, the place of sexual renunciation in religion, time and death. In introducing such weighty subject matter, Augustine certainly did not want readers to treat his Confessions lightly. They might be attracted to pick up the book out of curiosity or interest (and, both skilled in rhetoric and the psychology of human nature, he knew how best to entice them to do so).
The interest of Augustine in the operations of the human mind was revealed in his spiritual unease and his questioning of himself. This is seen in his Confessions, and in his images of the Trinity in his other great major work, De Trinitate ("On the Trinity"). This had a profound influence on the psychological theories of medieval scholars.
Through the medieval period and until the reintroduction of the ideas of Aristotle, the thoughts of Augustine on psychology were the only accepted ones. Augustine supported the view of Plato that the soul is immaterial and would never die, and that the body is material and mortal. He believed that knowledge was obtained through an awareness of self, and not from sensory impressions.
He thought of the mind as a unity with independent facilities (reason, memory, will and imagination) and originated what is now known as faculty psychology. He anticipated by many centuries the French philosopher, Rene Descartes, concerning the proof of self-existence: to doubt is to think, to think is to exist. The personality of Augustine himself was both complex and profound. He was a person who thought rather than a scholar who possessed a metaphysical, speculative mind. Very possibly his emotional and psycho-social maturation did not keep pace with his mental development.
This might go some way to explaining psychologically his need for friendship, some of the bad behaviour of his youth, and even his choice of community life after his conversion. The observations and descriptions by Augustine about human motives and emotions, his depth analyses of will and thought in their interaction, and his exploration of the inner nature of the human self have established one of the main traditions in European conceptions of human nature, even down to our own days.
In psychology, it is from Augustine that Europeans derived their sense of society, individuality and personal rights within the social order. His view on the role of history in humanity, and indeed his world view generally, dominated the thought about these matters in the Middle Ages. Certainly, it is impossible to describe the Middle Ages (and even subsequent centuries) fairly without giving Augustine a prime position.
The place of Augustine in the history of Psychology. He is given a full coverage in Chapter VI of this scholarly essay. http://psychclassics.yorku.ca/Baldwin/History/chap1-6.htm