Whoever honors his father atones for sins.It makes perfect sense. Every sin is a rejection of God's Fatherhood over us. We reject His loving, all-wise will to do our own thing. It's sin. This disfigures our soul, which is supposed to be taking on the image of His only begotten, Jesus. When we honor our parents, the earthly image of God put here by Him to guide us, we are in effect honoring God's Fatherhood over us and thus taking steps to undo the effects of our disobedience in our souls. In this way, we not only repair, or "atone," for sins; but we also "preserve" ourselves, take steps to protect ourselves, from falling into sin in the future. This honor we show to our parents is the triumph of Christ's grace within us, a manifestation of His obedience to Mary and Joseph out of obedience to the will of the Father! By Christ's power within us, we are overcoming sin and taking on the image of the Master.
and preserves himself from them....
Even if [your father's] mind fail, be considerate of him;
revile him not all the days of his life;
kindness to a father will not be forgotten,
firmly planted against the debt of your sins
— a house raised in justice to you.
(Sirach 3:3, 12-15)
The author of Sirach knew that the Lord sometimes calls us to atone for sin in this way while working against strong resistance. It can be somewhat easy to honor our parents in the normal circumstances of life, but we really have to overcome ourselves when we are called to care for a parent who fights our efforts, such as one suffering from dementia. This is when we really overcome the disfigurement sin has wrought in our souls via the grace-filled exertion it takes to remain firmly patient and kind in serving a parent. It cannot be done without Christ's grace, without Him loving our aging parents through us. We are called to become images of His own sacrificial love. Spiritually-speaking, this is about as lofty as it gets. Humanly-speaking, however, it feels horrible. It is an act of the will, made in union with Christ, the Ever-Faithful Son. [Believe me, I'm speaking to myself in this post.] The positive effects upon our souls are very real though — that is God's promise.
Addendum: If your Old Testament is missing the Book of Sirach, then this post is for you.