Both of these Cantatas were written for the Weihnachtstage of 1724, so Christmas. BWV 121 is on a text by Luther, 133 on a text by Caspar Ziegler. They both cover subjects appropriate for Christmas time.
The BWV 133 is about the joy of the arrival of God when Christ is born. It expresses a very intimate religious joy. This text has nothing to do with the prescribed readings for that day (Christ is higher than the angels, from Epistle to the Hebrews and Hymn to the word from the Gospel of John).
The opening chorale is a version of what Bach wrote in other works. He used it in two other places. This self-plagiarism is common in that time.