John Gill Commentary Numbers 15:28

"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." — Numbers 15:28 (ASV)

And the priest shall make atonement for the soul that sinneth
ignorantly
By offering his sin offering for him:

when he sinneth by ignorance before the Lord ;
to whom it is known to be such, before whom all things are naked, open, and well known:

to make an atonement for him, and it shall be forgiven him ;
upon that atonement made by sacrifice; so the forgiveness of the sins of all the Lord's people proceeds upon an atonement made by the blood and sacrifice of Christ: full atonement of sin and free forgiveness are not contrary to each other.