The Crown of Thorns, the "German" Cross and the Tablets - Symbol of Misery, Redemption and Thankfulness - the three basic themes of the Catechism. (The tablets stand for the Ten Commandments, which appear in the Catechism where it teaches that obedience is the proper form of thankfulness.
The Two Lights and the Fire - The Trinity - with the Hebrew name of God on the left orb, the Greek monogram for Jesus on the right orb, and the flame standing for the Holy Spirit. These is a long discussion of the Trinity in the Catechism.