Barth isn’t generally known for saying something with an economy of words. But this sentence on the nature of sin seems to get it just right:
[S]in is that interchanging of God and man, that exalting of men to divinity or depressing of God to humanity, by which we seek to justify and fortify and establish ourselves.
Karl Barth, The Epistle to the Romans, 6th ed., 190.
As for grace:
Grace is the act of God by which the new man shall be and is, and by which also he is free from sin. Our negative, known, human existence, so little conformed to Jesus, is filled with hope by the positive and secret power of the resurrection.