This is the last line of “By the Waters of Babylon”, Psalm 136, which is sung only three times a year, in matins for the three Sundays immediately preceding Great Lent.
Why do we sing this psalm then? The key to the answer is in the last line. The purpose of Great Lent is to change. Psalm 136 perfectly describes the demeanor we should have, but lamentation and longing for blessedness will not in themselves make us change. This psalm gives PRACTICAL advice about how to change.
“Blessed shall he be who shall seize and dash thine infants against the rock.” (Psalm 136:8)
I will early destroy all the wicked of the land; that I may cut off all wicked doers from the city of the LORD. (Psa 100:8 Sept)
” Keep back thy servant also from presumptuous sins; let them not have dominion over me: then shall I be upright, and I shall be innocent from the great transgression.” (Psa 19:13)
Let us understand clearly that as soon as we have any attachment to a bad thought (or “presumptuous sin”), we have already committed a sin , but that all is not lost if we can somehow battle this sin so that it does not lead us further along and cause us to commit the “great transgression” - to act on our thought. This “great transgression” may be a physical act, or it may be when we completely give in to a thought, and wallow in it like a glutton.
One “presumptuous sin” we must especially guard against is that our secret thoughts cannot harm us, or are “not as bad” as actually acting on the thought. If we do not dash the infant though against the rock as soon as possible, it will grow too big for us to control, and we will be thrown headlong into further sin. We must also remember that to the Lord, attachment to a sin is as the sin itself:
“Ye have heard that it was said by them of old time, Thou shalt not commit adultery: (28) But I say unto you, That whosoever looketh on a woman to lust after her hath committed adultery with her already in his heart.” (Mat 5:27-28 KJV)----------For a detailed exposition of Psalm 136, Listen to the catechetical talk on "By the Waters of Babylon"
This talk may also be found with other talks at http://www.orthodox.net/catechism