Большой русско-английский фразеологический словарь
Ф14
ФОМ А НЕВЕРНЫЙ (НЕВЕРУЮЩИЙ)NPsing onlyfixedWO
a skeptical person who is hard to convince of something
a doubting Thomas.
From the Biblical account of the apostle Thomas, who did not believe that Christ had risen from the dead until he saw and touched the wounds (John 20:24-29).