His power as a preacher was especially shown by his Licht des Lebens (Hanover, 1858) and Salz der Erde (1864).
In behalf of religious instruction he wrote his Lehrbuch der Religion fur die oberen Klassen protestantischer Schulen (Hanover, 1839; 9th ed., 1888), and later collaborated on the ill-fated new catechism of 1862.
At the same time, Petri was firmly opposed to any amalgamation of the Lutheran and Reformed Churches, and assumed an unfavorable position even toward the Inner Mission.
[1] His opposition to all movements in favor of a union of Lutherans and Reformed found renewed expression in his Beleuchtung der Göttinger Denkschrift zur Wahrung der evangelischen Lehrfreiheit (Hanover, 1854), an attack on the unionistic sympathies of the theological faculty of Göttingen.
After this, Petri withdrew more and more from public life; and the only noteworthy work which he subsequently wrote was Der Glaube in kurzen Betrachtungen (4th ed., Hanover, 1875).