Okay, so there was this guy named Alexamenos a very long time ago. And he believed in a man called Jesus who could do really amazing things like heal sick people and walk on water. But some people didn't like Jesus and they did bad things to him, like hitting him and ultimately killing him.
Now, after Jesus died, a lot of people continued to believe in him and worshiped him as the son of God. But some other people didn't like that either, and they made fun of those who believed in Jesus.
One of those people who made fun of Alexamenos was mean enough to draw a drawing (we call it a graffito) on the wall of a building where Alexamenos could see it. In the drawing, there was a man with a donkey's head hanging on the cross, and next to him was a man bowing down to worship him. The writing next to the drawing said, "Alexamenos worships his God".
Now, this was a very mean thing to do, but Alexamenos didn't give up on his belief in Jesus. He knew that what he believed was true, and he didn't care what other people said about him. And that's a very important lesson for all of us - to stay true to ourselves and what we believe, even when other people make fun of us.