In early Christian literature, particularly in Gnostic texts, we are introduced to a man called Didymus Judas Thomas.
A branch of Christianity, defined in the context of Gnosticism, claims that the man Didymus Judas Thomas was Jesus' twin.
A group of Christians called the Thomasine Christians may have believed that Didymus Judas Thomas was actually the Apostle Thomas and that Thomas and Jesus were blood twin brothers.
The beliefs have cast doubt on Jesus' death and have led to the substitution hypothesis or twin hypothesis.
This hypothesis attempts to provide a rational explanation for Jesus' death and resurrection. The notion here is that a twin may have impersonated Jesus after his death or taken Jesus' place on the cross.
In ancient texts, the Thomas Writings, Jesus is supposed to have referred to Thomas as his "twin." However, the writings remain vague and the exact definition of "twin" cannot be deduced. The word "twin" here may very well be symbolic.
The Thomas Writings include three important texts, The Gospel of Thomas, Book of Thomas the Contender, and the Acts of Judas Thomas.
While mainstream Christianity may have rejected these texts, the beliefs persisted in early forms of Thomasine Christianity that were prevalent at the time in Syria, Mesopotamia, and India.
Interestingly, there is another text called the Infancy Gospel of Thomas which purports to serve as a childhood biography of Jesus. While the author of this text remains unknown, it is believed that the author here was also Thomas. The rational explanation here was that Thomas as the brother of Jesus could clearly write about Jesus" childhood as he was a member of his family.
The Infancy Gospel of Thomas is also considered to be inauthentic and heretical in mainstream Christianity.