1 The keys are an office and power given by Christ to the Church for binding and loosing sin, not only the gross and well-known sins, but also the subtle, hidden, which are known only to God, as it is written in Ps. 19:13: Who can understand his errors? And in Rom. 7:25 St. Paul himself complains that with the flesh he serves the law of sin.
2 For it is not in our power, but belongs to God alone, to judge which, how great, and how many the sins are, as it is written in Ps. 143:2: Enter not into judgment with Thy servant; for in Thy sight shall no man living be justified.
3 And Paul says, 1 Cor. 4:4: For I know nothing by myself; yet am I not hereby justified.