Holy Trinity Romanian Orthodox Church was designed by famous neo-classical architect William Strickland (1788 -1854), one of the founders of the Greek Revival movement in the United States. Thus, St. John’s/Holy Trinity is now Strickland’s oldest surviving building, and the first of the only two churches he has built in Philadelphia. The other one is St. Stephen's Episcopal Church (1822-1823), at the intersection of Tenth and Market.
Two major periods are reflected in the history of this church. During its first 100 years (1816-1919), the church belonged to and was used by the Episcopal congregation exclusively. The second period, which began in 1919, is the period when the building became crucial for the spiritual and cultural identity of the Romanian Orthodox immigrants.
Altough the building has been functioning as an Orthodox church, for almost one hundered years, its principal original features have been preserved virtually intact. Thus, one could say that Holy Trinity does not look like a traditional Orthodox church. Both the interior and the exterior of the church embody a unique combination of early American architectural history and the religious and immigration history of the neighborhood. The only significant changes and additions to the building occurred before 1931, when the church was consecrated as an Orthodox sanctuary, so that the space could function properly according to its canons. A short wooden steeple with a cross was added over the northern entrance, and functions as a bell tower with three bells. An embellished icon screen, called iconostasis, separating the altar from the nave was also added, and the interior walls were painted in a vivid decorative style.