John Calvin Commentary 2 Peter 1:9

"For he that lacketh these things is blind, seeing only what is near, having forgotten the cleansing from his old sins." — 2 Peter 1:9 (ASV)

But he that lacketh these things. He now expresses more clearly that those who profess a naked faith are wholly without any true knowledge. He then says that they go astray like the blind in darkness, because they do not see the right way which is shown to us by the light of the gospel. This he also confirms by adding this reason: they have forgotten that through the benefit of Christ they had been cleansed from sin, and yet this is the beginning of our Christianity. It then follows that those who do not strive for a pure and holy life do not understand even the first rudiments of faith.

But Peter takes this for granted, that those who were still rolling in the filth of the flesh had forgotten their own purgation. For the blood of Christ has not become a washing bath to us, that it may be fouled by our filth. He, therefore, calls them old sins, by which he means that our life ought to be formed differently, because we have been cleansed from our sins; not that anyone can be pure from every sin while he lives in this world, or that the cleansing we obtain through Christ consists of pardon only, but that we ought to differ from the unbelieving, as God has separated us for himself.

So, although we daily sin, and God daily forgives us, and the blood of Christ cleanses us from our sins, yet sin ought not to rule in us, but the sanctification of the Spirit ought to prevail in us; for so Paul teaches us in 1 Corinthians 6:11, And such were some of you; but ye are washed, etc.