Trinitatis VIII is the 8th Sunday after Trinitatis. Three cantatas for this day.
Erforsche mich, Gott, und erfahre mein Herz, BWV 136, is from the first cycle, so only a few months after Bach arrived in Leipzig. He found out that the requirement for a weekly cantata (or more often) was a serious burden for a composer, especially if the cantatas were expected to be in two parts, one part before and one after the sermon. So he created a series of slightly simpler cantatas, for which he also used older compositions as inspiration.
Wo Gott der Herr nicht bei uns hält, BWV 178, is from the second cycle, so it''s a chorale cantata. This cantata is based on a hymn from 1524 by Justus Jonas (1493-1555), preacher, professor and colleague reformer of Martin Luther.
Es ist dir gesagt, Mensch, was gut ist, BWV 45, dates from 1726, when Bach did not write a weekly cantata anymore. Just like last Sunday, this cantata is based on a text by Duke Ernst Ludwig von Sachsen-Meiningen, who wrote a series of cantata libretto''s in 1705. These texts were used previously by Bach''s cousin, Johann Ludwig Bach, but he liked the texts so much he wrote 7 cantatas based on these lyrics.