John Calvin Commentary Joshua 9:14

"And the men took of their provision, and asked not counsel at the mouth of Jehovah." — Joshua 9:14 (ASV)

And the men took of their victuals, etc. Some commentators here resort to unconvincing fabrications that they ate the bread to determine from the taste whether it was stale from age, or that they confirmed the covenant by a feast. The words, in my opinion, are rather an indirect censure of their excessive credulity in having, on slight grounds, acquiesced in a fabricated narrative, and in having paid attention merely to the bread, without considering that the fabrication lacked credibility.

And certainly, had their senses not been dulled, many things would have instantly occurred to them to refute the Gibeonites. But as it sometimes happens that the most piercing eyes are dazzled by an empty spectacle, they are more severely condemned for not having determined God's will. The remedy was at hand, had they simply consulted the oracle before attempting anything. It was a matter deserving of careful inquiry, and it was therefore a sign of gross carelessness—since a priest was ready to seek an answer from God by means of Urim and Thummim—to decide rashly in an obscure case, as if they had no means of obtaining advice. Their rashness was all the more inexcusable since it was combined with such supine neglect of the grace of God.