Charles H. Spurgeon on Psalms 130:3

If thou, LORD, shouldest mark iniquities, etc. But doth not the Lord mark iniquity? Doth not he take notice of every sin acted by any of the children of men, especially by his own children? Why, then, doth the Psalmist put it upon an if? "If thou, LORD, shouldest mark iniquity." 'Tis true, the Lord marks all iniquity to know it, but he doth not mark any iniquity in his children to condemn them for it: so the meaning of the Psalm is, that if the Lord should mark sin with a strict and severe eye, as a judge, to charge it upon the person sinning, no man could bear it.

The word rendered to mark notes, first, to watch, or to observe with strictest diligence, and is therefore in the noun rendered a watch tower, upon which a man is placed to take observation of all things that are done, and of all persons that pass by, or approach and come near. A watchman placed upon a high tower is bound industriously and critically to observe all passengers and passages, all that his eye can reach. So saith the text, —If thou shouldest mark as a watchman, and eye with rigour everything that passeth from us, "who shall stand?" that is, make good his cause in the day of his judgment and trial before thee.

Secondly, the word signifieth to keep in mind, to lay up, to have, as it were, a store and stock, a memorial or record, of such and such things by us. In that sense it is said (Ge 37:11), "Joseph's brethren envied him; but his father observed the saying": he marked what Joseph spake about his dreams, he laid it up, and did not let it pass away as a dream, or as a vision of the night. Thus, by "If the Lord should mark iniquity", we understand —if he should treasure up our sins in his memory, and keep them by him, "who were able to stand when accounted with?" The Lord, in a way of grace, seeth as if he saw, not, and winks at us oftentimes when we do amiss. —Joseph Caryl.