It is a sin to know the good you need to do, and you don't do it.

 James 4:17

If anyone, then, knows the good they ought to do and doesn’t do it, it is sin for them.

It is a tough one. Basically, when the truth have been revealed, and you know the good that you need to do, but you don't do it, that is sin. This means, the moment it is made known to you what is right and what is wrong, you can't turn a blind eye and pretend as though you didn't know. God sees everything and we have nowhere to hide. 

We can either see this as a harsh thing, or we may use it as motivation. For example, in Genesis 12:2 God was telling Abraham that He will bless him and so he will be a blessing to others. As the descendants of Abraham, by faith, we also have this blessing, which we should claim for ourselves. Therefore, when God starts blessing you, because you would've known already that God is commanding and expecting you to bless others, don't see that as a burden, recognize it as a fulfilled promise (that eventually you are blessed) and in turn, to show gratitude to God and to be in obedience to His command, you bless others, with a cheerful heart. But, what the bible verse above highlights is that should you then be too greedy, and not bless others with what God has blessed you with, then you will be committing a sin.

Our prayer should be, for the Lord to give us the Spirit to nudge and remind us to bless others when the blessing arrives; to give us the Spirit to discern who to bless and how to bless them. Thus, remembering that God is always strategic, and He has already told us about the seeds that can be planted in different environments where some environments are not conducive to grow the seed, whilst other environments have good soil that will bear much fruit (Matthew 13:18-23).