According to you a Christian is ‘sinner and righteous at the same time’. How is that possible? It is one or another, is not it?

Luther:

At first I thought so. Each time I sinned, I went to the priest and my sins were forgiven. At that moment I was again a righteous person, I thought. But a little later I again sinned and I was again a sinner. This pattern continues, and so I wandered back and forth between false certainty on the one hand (as if sin was behind me), and desperation on the other hand (as if the grace had been for nothing).

Until I saw that I was not sinner and righteous successively, but at the same time: sinner in reality, but righteous in hope. It is like a patient who is promised by the docter that he shall recover, he believes it and he does what the docter says to recover in reality. Is he already healthy then? No, he is ill and healthy at the same time. Ill in reality, but healthy according to the promise of the docter and in hope. The docter counts him as healthy, because he shall be. He promises that the illness shall not end in death.