If someone thinks he knows something, he does not yet know to the degree that he needs to know. (1 Corinthians 8:2)
This means we never fully arrive at a truth from God in this life. There is always more to learn from God on any given thing and we should humbly learn from God.
I will always remind you of these things, even though you know them and are firmly established in the truth you now have. (2 Peter 1:12)
Peter knew the importance of God's truth and that even if we know and stand firm in the truth we still need to be constantly reminded of it. Because of the infinite value of God's truth we can always benefit from it in all ways.