Good question, Max. You have an eye for English usage. I'll try to make this manageable of length. Yes, it SOUNDS incorrect because it is uncommon and yet is acceptable, even proper, for the following reasons. "Come" in this instance is a past participle (come, came, have come) acting as an adjective. As a verb, we precede them with 'have' and as adjectives, with a form of 'to be'. The verse could read, "Suppose I HAVE now come..." This would express an action taken. "I AM now come..." expresses the condition of the one who has just come. Follow? We have countless expamples in everyday speech. "I have hurt myself; I am hurt". "You have filled the bottle completely; it is completely filled". Now let's turn it around. When someone thanks you, do you say, "You have come well [to me]", or do you say "You are welcome."? This form is so time worn and frequently used as to be almost unrecognized, but it is identical to the form in question. Italian past participles even come in both varieties, those preceded by 'to have' and those which follow 'to be'. It seems that the bigger the word, the more acceptable following 'to be' sounds. I am overwhelmed. You are were terrified. She is flabbergasted. The city will be inundated. You get the point. Jesus is quoted [another example] in many translations as saying, "I am come to do your will" and taught his follower to pray "Hallowed be thy name". OK. I am finished. (I have finished? Take your pick.)