大乘入道次第開決 1.109

1.109

能深忍樂清淨等者。忍謂忍可。即是信因。由因忍可方生信故。樂謂樂欲。即是信果。由因有信方樂欲故。性自證清能淨心等。如水清珠能清濁水。故清淨性名之為信。

Regarding "being able to deeply approve, take delight in, and be pure, etc.": "approval" means assent, which is the cause of faith, because only through assent does faith arise. "Delight" means desire, which is the fruit of faith, because only through faith does desire arise. Its nature is intrinsically pure and can purify the mind and so forth, like a water-clearing jewel that can clarify muddy water. Therefore, the nature of purity is called "faith."