John Calvin Commentary Acts 28:5

"Howbeit he shook off the creature into the fire, and took no harm." — Acts 28:5 (ASV)

Shaking off the viper. The shaking off of the viper is a sign of a calm mind. For we see how greatly fear troubles and weakens people. Yet, you must not think that Paul was entirely without fear.

Faith does not make us unfeeling, as foolish people imagine when they are out of danger. But although faith does not entirely take away the awareness of dangers, it tempers this awareness, ensuring the godly are not more afraid than is fitting, so that they may always be courageous and have a good hope.

So, even though Paul understood that the viper was a venomous creature, he still trusted in the promise made to him and did not fear its venomous bite so much that it distressed him, because he was even ready to die if it had been necessary.