Matthew Henry Commentary Numbers 15:22-29

"And when ye shall err, and not observe all these commandments, which Jehovah hath spoken unto Moses, even all that Jehovah hath commanded you by Moses, from the day that Jehovah gave commandment, and onward throughout your generations; then it shall be, if it be done unwittingly, without the knowledge of the congregation, that all the congregation shall offer one young bullock for a burnt-offering, for a sweet savor unto Jehovah, with the meal-offering thereof, and the drink-offering thereof, according to the ordinance, and one he-goat for a sin-offering. And the priest shall make atonement for all the congregation of the children of Israel, and they shall be forgiven; for it was an error, and they have brought their oblation, an offering made by fire unto Jehovah, and their sin-offering before Jehovah, for their error: and all the congregation of the children of Israel shall be forgiven, and the stranger that sojourneth among them; for in respect of all the people it was done unwittingly. And if one person sin unwittingly, then he shall offer a she-goat a year old for a sin-offering. And the priest shall make atonement for the soul that erreth, when he sinneth unwittingly, before Jehovah, to make atonement for him; and he shall be forgiven. Ye shall have one law for him that doeth aught unwittingly, for him that is home-born among the children of Israel, and for the stranger that sojourneth among them." — Numbers 15:22-29 (ASV)

Though ignorance will to a degree excuse, it will not justify those who might have known their Lord's will, yet did not do it. David prayed to be cleansed from his secret faults, those sins of which he himself was not aware.

Sins committed ignorantly will be forgiven through Christ the great Sacrifice, who, when he offered himself up once for all on the cross, seemed to explain one part of the intention of his offering in that prayer: Father, forgive them, for they know not what they do. It was a favorable sign for the Gentiles that this law of atoning for sins of ignorance was expressly made to extend to those who were strangers to Israel.