"the Lenten fast ... may be fulfilled ... by the renunciation of sin"

"whatever degree of progress has been made there is no one who should not be more advanced. All must therefore strive to ensure that on the day of redemption no one may be found in the sins of his former life.
Dear friends, what the Christian should be doing at all times should be done now with greater care and devotion, so that the Lenten fast enjoined by the apostles may be fulfilled, not simply by abstinence from food but above all by the renunciation of sin."

from a sermon by Saint Leo the Great, pope