Approaching the main entrance of the church, are two low steps which lead, at once, into the narthex. The upper portion of the narthex screen is constructed of transparent glass, which permits the visitor to survey the interior with a single glance. No ceiling above supporting the customary accepted choir loft oppresses the visitor and, instead, he is impressed by the expansiveness of the nave, unfolding itself into the choir and sanctuary. Four small wood carved crosses break the monotony of the ridge of the narthex screen. Fastened against the south wall of the narthex, is a beautiful bronze-edged Honor Roll on which are inscribed the names of the servicemen and women of the parish. Against the walls on either side of the doors, are oak tablets bearing a partial list of the donors and benefactors. The floor of the narthex, as well as the three aisles, is laid the donors with off-shade brown and red tile, which give an appearance of permanence and beauty. The confessionals are formed from a continuation of the narthex screen located in proper liturgical style on each side of the main entrance. A half door in the center of these, covered with a penitential shade of burgundy velvet draping, adds a richness to their appearance.
The nave was planned to accommodate approximately 350 people. In it the pews are a bench type open back of red oak to further the rustic atmosphere of the edifice. The walls are of roughly finished plaster left unpainted. They are divided into equal sections by strong laminated arches, which support the roof. The ceiling is of celotex, in a natural tone, and the rafters and cross pieces are clearly visible, giving the appearance of strength and sturdiness and, at the same time, symbolizing the inverted bark of St. Peter. The only flash of color, provided in the nave, comes from the exterior light against the stained glass windows, and the subdued gold and blue on the wood carved Stations of the Cross.
Suspended from the ceiling on circular chains, are two rows of ornamental church lanterns fashioned from Swedish wrought iron and delicately marked with red medallions to supply a dash of color. Their design is likewise Gothic, in character.
Hearkening back to the Medieval spirit of the liturgy, the choir stalls are located between the nave and the sanctuary, lifted on either side in a slight elevation. An attractively wrought iron communion rail, with its table of light oak, makes the division apparent.
The sanctuary provides ample space for all solemn occasions, or through the entrance on the right, which leads directly into the sacristy. Here, one stands before the beautiful liturgical altar, resting on a solid predella of deep black terrazza. The stepes, or supports to the table, are a Nubian shade of marble. The background of the underpart is a highly polished Tennessee gray marble, in the center of which is the symbol XR "CHI RHO" wrought in bronze and offset by black marble. The table surface of the main altar is hewn from a lighter vein of the same material. Two filuted oaken columns rise gracefully above the altar, to support an ornate canopy trimmed with cardinal velvet and fringed with gold. On the frontis-piece, of the canopy, are three heraldic shields. The highest of these depicts the coat-of-arms of the present gloriously reigning Pontiff, Pope Pius XII, during whose pontificate the church was dedicated. The one on the right marks the episcopate of the Most Reverend John Mark Gannon, Bishop of Erie, who created the parish. The third is emblematic of the ancestral shield in the background, while on either side of the altar riddle posts of wrought iron support a similar fabric, and at the same time, hold the Sanctus candle.
A free standing tabernacle of hand wrought bronze adorns the center of the altar. Its distinctiveness has been preserved by, and exquisite design for, the revolving door, which depicts the vine and the wheat and the head of the lion, all symbols of the Blessed Sacrament and of St. Mark, the patron of the parish. Boldly inscribed above these is the word "Sanctus", in capital lettering, proclaiming the sanctity of this precious article. Rising majestically behind the tabernacle, is a cast bronze crucifix of the vested Christ wearing his regal and sacerdotal robes. Six hand-wrought bronze candlesticks, designed especially for this parish, stand as silent sentinels along-side the tabernacle. On the Gospel side of the altar, standing on the floor of the Sanctuary, is an artistic sanctuary lamp over five feet high, in whose floral cup burns the white colored light indicative of the Blessed Sacrament. Chiseled close by in the wall of the sanctuary, is an oaken ambry with the words Olea Sacra inscribed on the door. On either side of the choir, but in complete view of the worshipper, are two oratories. In the one on the north, is a delicately carved statue of Our Lady, Mediatrix of all graces. Behind the wood carving, is suspended an exquisitely embroidered drapery gently falling from a canopy made of oak. Since there is not sufficient depth to permit an altar, this oratory takes on the nature of a shrine. On the south side, however, an altar has been erected where Mass can be celebrated, but as correctly planned, no provision is made to reserve the Blessed Sacrament there. On this altar before the reredos, stands a wood carved statue dedicated to St. Joseph, the laborer, and the same type of fabric provides a beautiful background.