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. 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. And Paul, 1 Cor. 4, 4, says:
For I know nothing by myself; yet am I not hereby justified.