Peace be with you!
This is not the wish for hoped for future, but the declaration that, in our midst, he is our present, our peace; this is the only thing that allows the disciples to pass from fear to joy. Jesus repeats this greeting, because, in the joy of peace, every disciple can go out on the mission that expands the presence of the Risen Christ, in the sign of the Spirit that brings mercy and forgiveness, just as at the beginning of history He brought the breath of life and beauty into the midst of chaos.
Thomas was not with them when Jesus came. Nor were the generations of believers who have faith in the Word received from those witnesses. This is why the episode of Thomas is significant. In the presence of the eyewitnesses (We have seen the Lord!), Thomas asks for visible and tangible proof: If I don't see and if I don't put my finger, I won't believe.
Jesus comes again eight days later, apparently just for him, Thomas. This is a new act of mercy that allows Thomas too to see—and even more: to touch, literally, the wounds of the Passion. Now, Thomas, also on behalf of believers who will come later, makes his beautiful confession: My Lord and my God! The blessedness of those who "have not seen and have believed" is also a grace, a gift received, and the faith accepted through listening.