Didymus means “twin” or “double” which indicates that Thomas had a twin brother.

The disciples are still called “the twelve” though Judas is now dead (Matt 27:1-5).

Matthew 27:1-5  Early in the morning, all the chief priests and the elders of the people came to the decision to put Jesus to death.  They bound him, led him away and handed him over to Pilate, the governor.  When Judas, who had betrayed him, saw that Jesus was condemned, he was seized with remorse and returned the thirty silver coins to the chief priests and the elders.  “I have sinned,” he said, “for I have betrayed innocent blood.” “What is that to us?” they replied. “That’s your responsibility.”  So Judas threw the money into the temple and left. Then he went away and hanged himself.

Thomas was not with the other disciples when Jesus made His first appearance to them. We are not told the reason for his absence, but there is no blame put on him for it so it would seem that his reason was legitimate. Nevertheless, his absence from the assembling of the other believers cost Thomas the satisfaction of seeing his Master risen from the dead.

We never know how much we will miss when we don’t meet with our fellow believers. Often sorrow, pain, and despondency drive us to shut ourselves up alone and not seek the fellowship of our Christian friends, but those are the very times when we need each other, and those are the times when we are most likely to see Jesus face to face (Heb 10:25, 1Th 5:11, Mt 18:20).

Hebrews 10:25  Let us not give up meeting together, as some are in the habit of doing, but let us encourage one another—and all the more as you see the Day approaching.

1 Thessalonians 5:11  Therefore encourage one another and build each other up, just as in fact you are doing.

Matthew 18:20  For where two or three come together in my name, there am I with them.”