大乘起信論裂網疏 5.81

5.81

Second, as for explaining the arousal of the mind through understanding and practice: in terms of the complete cultivation according with nature, faith itself is understanding, is practice, and is realization. Hence it is said: "The initial letter A already possesses the merits of all letters." Even though forty-two stages are distinguished, it is like entering the sea to discuss shallowness and depth—both shallow and deep are the sea. How could there be faith without understanding, practice, and realization? How could understanding, practice, and realization be made to reside strictly after faith? It is solely because the real stages are difficult to fathom, whereas relying on the provisional makes them easy to clarify, that understanding and practice are distinguished following faith.

The text has two parts: first, clarifying the stages; second, clarifying the practices. Now, the first.