"But perhaps you will become a great artist": do you think the opinion of the priest about the boy is appropriate?

The priest's opinion would seem to be correct. Little Joji spends practically every spare moment drawing pictures of cats. It's almost an obsession with him, so much so that it's pretty much all he does. His parents know that he wouldn't be much use as a farmer; and the priest soon realizes that Joji has no future in his line of work, either. It would appear that the boy's cut out to be an artist and nothing else.
After his incredibly scary experience at the temple, Joji is more determined than ever that, for as long as he lives, no one but no one will ever stop him from drawing cats. And Joji is as good as his word, becoming a great artist, honored throughout the whole country for his drawings of—wait for it—cats. So the priest was right.