Doubting Thomas is a term used to describe someone who will refuse to believe something without direct, physical, personal evidence; a skeptic.
I’ve used this term many times, and had no idea that we get this expression from Christianity! Live and learn.
Thomas the Apostle claimed he wouldn’t believe in the resurrection until he actually touched the wounds of Jesus. Jesus understood Thomas’ doubt and invited him to do just that. But according to the Gospel of John, Jesus also included this gentle rebuke: blessed are they that have not seen, and yet have believed. (John 20:29)
Origin courtesy of Merriam-Webster, and it can be found here. Thanks to Kim for bringing it to my attention.