A. ist ein Virtuose und der Himmel ist sein Zeuge.
A. is a virtuoso and heaven is his witness. [Kaiser/Wilkins]
A. is a virtuoso, and Heaven is his witness. [Hofmann]
We could take this as an observation one person makes about another, but in that case, how would we be in a position to designate heaven a witness? I assume that virtuoso means more than an expert musician, but a virtuous person. For the Greek philosophers and mythologers, any excellence was the signature of some god or other; if we think of virtuosity this way, then you can't be a virtuoso unless the gods allow it. This is much like the weird Christian idea of grace. It amounts to saying that even moral excellence can't be imputed to you, but only bestowed on you from its source, which, at least to me, eliminates you from consideration altogether. You can't even argue that you received excellence because you deserved it, since deserving it would mean being excellent on your own; if it's possible for you to be excellent on your own, then any divinely-bestowed excellence would be superfluous, and if you can't be excellent on your own, then heaven bestows excellence on some other basis, or no basis.
How can you know that you are virtuous? You can try to be good, but how do you know if you're succeeding? Kafka doesn't say "virtuous," he says "A. is a virtuoso," which implies skill. If heaven witnesses skill, and if witnessing implies approval, then what matters isn't moral attainment but skillfulness, which is consistent with other aphorisms.