The account in 2 Nephi 16 (and Isaiah 6) is interesting.
1 In the year that king Uzziah died, I saw also the Lord sitting upon a throne, high and lifted up, and his train filled the temple.
2 Above it stood the seraphim; each one had six wings; with twain he covered his face, and with twain he covered his feet, and with twain he did fly.
3 And one cried unto another, and said: Holy, holy, holy, is the Lord of Hosts; the whole earth is full of his glory.
4 And the posts of the door moved at the voice of him that cried, and the house was filled with smoke.
5 Then said I: Wo is unto me! for I am undone; because I am a man of unclean lips; and I dwell in the midst of a people of unclean lips; for mine eyes have seen the King, the Lord of Hosts.
6 Then flew one of the seraphim unto me, having a live coal in his hand, which he had taken with the tongs from off the altar;
7 And he laid it upon my mouth, and said: Lo, this has touched thy lips; and thine iniquity is taken away, and thy sin purged.
Certainly we see the atonement in here. But Isaiah knew Christ far better than I do. As one of the most notable of all prophets I am sure he understood the atonement many times better than I ever will. He was baptized before having this experience. He was worthy to be a prophet. It appears that from that alone he wasn't prepared for this. There was greater cleansing necessary beyond what was initially offered at baptism.
Just as we see in Isaiah's vision, the cleanliness offered at baptism isn't the full measure available through the atonement. Otherwise we wouldn't read such things as
Alma 13: 11 Therefore they were called after this holy order, and were sanctified, and their garments were washed white through the blood of the Lamb.
12 Now they, after being sanctified by the Holy Ghost, having their garments made white, being pure and spotless before God, could not look upon sin save it were with abhorrence; and there were many, exceedingly great many, who were made pure and entered into the rest of the Lord their God.