Doubting Thomas is a term that is used to describe someone who will refuse to believe something without direct, physical, personal evidence; a skeptic.
The term is based on the Biblical account of Thomas the Apostle, who doubted the resurrection of Jesus and demanded to feel Jesus' wounds before being convinced (John 20:24-29), although the Bible does not mention if actual contact took place. After seeing Jesus alive and being offered the opportunity to touch his wounds—according to the author of the Gospel of John—Thomas professed his faith in Jesus; on this account he is also called Thomas the Believer.
According to the Biblical account, Jesus then said "blessed are they that have not seen, and yet have believed", suggesting Jesus's preference was for faith over skepticism. Thomas was a very close friend of Jesus, but did not believe Jesus came back to life after he died on the cross.