Romans 3:20

Therefore; i.e. Seeing the Gentiles, by the law of nature, and the Jews, by the written law, are thus subject to the judgment of God; and seeing no one is able to fulfil the law, and satisfy for the breach of it; therefore, &c.

By the deeds of the law; he means the moral law, and not the ceremonial law only or chiefly; even that law that forbids theft and adultery, as Romans 2:21,Romans 2:22, and concupiscence, as Romans 7:1-25; and by which, as this text says,

is the knowledge of sin; to which Gentiles as well as Jews are obliged, and by which therefore they are condemned.

No flesh; a common synecdoche: see Genesis 6:3,Genesis 6:12, and elsewhere. The same with no man living, in the psalmist; especially being depraved with original corruption, which is called flesh in Scripture.

Be justified in his sight; or be discharged in the court of heaven: the phrase is taken from Psalms 143:2, see annotations there.

For by the law is the knowledge of sin: lest any should think that the law hereupon is useless, he goes on to show its use, but a quite contrary one to what they intended. It convinceth us of our guilt, and therefore is far from being our righteousness, Romans 7:7; 1 Corinthians 15:56.