Someone pointed out the other day that the only time Jesus is directly addressed in the nominative κυριος as opposed to the vocative κυριε is in Thomas’ declaration, “My Lord and my God” (John 20:28, ο κυριος μου και ο θεος μου). Is there any significance?
On the one hand, the nominative can be used to function as the vocative, so there is no necessary significance. And yet it is interesting that this is the only example of κυριος being used this way of Jesus. In every other case (as far as I can tell) it is κυριε.