Bruckner was Austrian and Catholic with the years he spent in Sankt Florian as a youth and later as a young adult were formative. He was a devout Catholic all his life and some of the first music he was acquainted with was sacred music. While he is more well known for his symphonies, he wrote a considerable amount of music for the church. At first his compositions reflected his younger days as it was traditional and conservative, but by the time he had completed his studies with Simon Sechter (whom he began lessons with after he had been composing for a few years) he had been exposed for the first time to the music of Wagner and Liszt.
He was slow to develop as a composer (he didn't write his first symphony until 1863 when he was 39 year old, and this symphony was an assignment from one of his last teachers). His progress as a composer can be seen clearly from his sacred compositions, the first which was written in 1836 up to the last written in 1892, but most of his vocal works differed from the symphonic works. The sacred works reflect Bruckner's faith in that they were written simply (and the later ones very skillfully) and reverentially.
The motet Os Justi Meditabitur was written in 1879 at about the same time as the 6th Symphony. It written in response to a group of church music reformers known as the Cecilians. This group of Catholic priests and musicians was in reaction to the modernization of church music since the Enlightenment. Bruckner's work is a motet that was intended as a gradual, a piece of music connected with the ritual of the Eucharist (Communion in the Protestant church). The motet is written in the key of F but uses no sharps or flats in either the key signature or the written notes, thus it is in Lydian mode, the fifth mode of eight in the system of church modes instituted sometime in the 8th century. The motet is for a usual four-part a capella choir, but there are two places in the work where it is written in eight parts. Bruckner dedicated the work to Ignaz Traumihler, who was choir director at St. Florian as well as a Cecilian. The text is taken from Psalm 36 in the Latin Vulgate, verses 30-31:The mouth of the righteous utters wisdom,
and his tongue speaks what is just.
The law of his God is in his heart:
and his feet do not falter.