Messiah is a series of yaoi games from Core (or more specifically Core Dusk). The title implies some sort of religious undertone or reference, but I'm not sure what that is. I haven't found any info as to what the storyline of the games actually is. However, I do know that the artwork for the games is done by the artist Carnelian, who is also known for her anime and hentai work.
The pictures in this post come from the artbook Trinity as well as the game Messiah: Paranoia Paradox.