In the New Testament
Thomas, called Didymus ("the Twin" — both his Aramaic and Greek names simply mean "twin," so it's not clear who his twin was or if the name is more symbolic than literal), appears sparingly but memorably in John's Gospel. When Jesus insists on returning to Judea despite the danger, knowing Lazarus has died, Thomas is the one who says, with a kind of grim loyalty, "Let us also go, that we may die with him" (John 11:16). At the Last Supper, his honest confusion prompts one of Jesus's central self-declarations: when Jesus says "you know the way to the place where I am going," Thomas objects, "Lord, we do not know where you are going, so how can we know the way?" — drawing the answer "I am the way, and the truth, and the life" (John 14:5–6). Most famously, Thomas was absent when the risen Jesus first appeared to the other disciples, and refused to believe their report without physical proof: "Unless I see the nail marks in his hands... I will not believe" (John 20:24–25). When Jesus appears again eight days later and offers exactly that proof, Thomas responds with the Gospels' most direct confession of Christ's full divinity: "My Lord and my God!" (John 20:28) — earning him the enduring, slightly unfair nickname "Doubting Thomas," though the text arguably shows him arriving at the clearest theological statement of anyone in the Gospel of John.
The apostle who went farthest: the Indian tradition
Thomas carries a uniquely well-attested and geographically significant later tradition among the Twelve. The apocryphal Acts of Thomas (composed in Syriac, likely second to fourth century) records him traveling to the court of the Indo-Parthian king Gondophares — a king whose historical existence has been independently confirmed through coin finds in Afghanistan, Pakistan, and a stone inscription (the Takht-i-Bahi inscription), lending unusual external corroboration to at least the plausibility of this part of the tradition. Early patristic witnesses independently attest to Thomas's connection with India: Origen (via Eusebius) records that Thomas's assigned mission field was Parthia, extended in later tradition specifically to India; Ephrem the Syrian (4th century) wrote hymns describing Thomas evangelizing "India's painful darkness"; and Gregory of Tours, writing around 590, records testimony of an eyewitness who had visited Thomas's tomb in India.
The Saint Thomas Christians of Kerala, on India's Malabar coast, maintain an unbroken living tradition — supported by their own ancient liturgical songs (Ramban Pattu, Margam Kali Pattu) — that Thomas landed at Muziris in AD 52, founded seven churches along the Malabar coast, later traveled to the Coromandel coast in the southeast, and was martyred by spearing near Mylapore (in modern Chennai) around AD 72, with his remains later transferred to Edessa and eventually to Ortona, Italy. This community's continuous existence, self-identifying as "Indian in culture, Christian in faith, and Syrian in liturgy," is treated by many historians as the strongest single piece of evidence for the broader apostolic tradition, since the community's own memory of its origin has been maintained without external imposition for many centuries, independently corroborated by later visitors including Marco Polo (1292) and various medieval travelers who record visiting Thomas's tomb.
Why it matters
If the Indian tradition is even broadly accurate, Thomas traveled and founded churches farther from Jerusalem than any other apostle whose destination can be reasonably reconstructed — meaning the "doubting" apostle became, on the strength of the very evidence he once demanded for himself, the one who carried the gospel to the farthest edge of the known world. It's also a valuable corrective to any assumption that African and South Asian Christianity are late colonial imports: the Mar Thoma Christians of Kerala have a plausible claim to a Christian presence in India nearly as old as Christianity itself in Europe.