He was sent to the Orient by his superiors, that he might eventually render service to the Roman Church, through his knowledge of the ecclesiastical documents and the positive theology of the Greeks.
Henceforth he was in communication with Greek scholars, notably Leo Allatius, Basil Falasca, George Coresi, Pantaleon Ligardio, and others.
After his return to Paris (16 July 1644) he devoted himself to putting in order the rich material he had brought from the East, which he had increased by visits to the libraries of France and Italy.
Goar's major work is his Euchologion sive Rituale Graecorum complectens ritus et ordines divinae liturgiae (Paris, 1647), a classic on Greek liturgy; it is important for its original texts and for its learned commentaries; in the second edition (Venice, 1730) a number of errors were corrected.
Finally, Goar wrote the "Historia universalis Joannis Zonarae ad manuscripts codices recognita" (Paris, 1687); it was continued and completed by Du Cange.