up
Explanation
In this comic, Jesus Christ has returned for the Second Coming and is addressing his followers. He announces that he will move "up" and, having gone into "up," will move around and then come "down" to instantly be elsewhere. Two of his disciples watch and whisper to each other: "Should we tell him?" / "No, just clap. He looks so happy."
The joke is that Jesus is describing movement in only two dimensions -- up and down, and then laterally -- as if the universe were a flat plane. He thinks moving "up" and then "down" somewhere else constitutes miraculous teleportation, when in reality he's just describing basic movement across a 2D surface. The caption confirms this: "To his lasting embarrassment, during the Second Coming, Christ had thought our universe was two-dimensional."
The humor operates on multiple levels. First, there is the absurdity of an omniscient deity having a fundamental misunderstanding of physics. Second, the disciples' reaction is very human and relatable -- rather than correcting someone who is confidently wrong and clearly excited about it, they decide to just politely clap along, a dynamic familiar to anyone who has watched a friend or authority figure proudly demonstrate something embarrassingly incorrect. The comic also plays with the intersection of theology and geometry/physics in a way that is characteristic of SMBC's nerdy sensibility.