“Therefore to him that knoweth to do good, and doeth it not, to him it is sin.”
King James Version (KJV)
4:17 Therefore to him that knoweth to do good and doeth it not - That knows what is right, and does not practise it. To him it is sin - This knowledge does not prevent, but increase, his condemnation.
Jas 4:17 To him that knoweth to do good, and doeth [it] not, to him it is sin. The connection of thought is probably as follows: "I have warned you that such boasting is wrong, and that the right way is to speak submissively. If you know how to do the right and yet do it not, the sin is the greater on account of your knowledge". See Jas 4:15.