What's shown in the image
The image shows a large face drawn in black and white: the eyes are half-closed and looking upward, the mouth is wide open showing the upper teeth, and the corners of the lips curl unevenly upward. The eyebrows and wrinkles are rendered in strokes, adding expressiveness to the face and making the emotion easy to read.
The facial expression is ambiguous: it conveys bliss, longing, and almost painful joy all at once. It's precisely this duality that let the face be used in all sorts of contexts, from delight over food to outright irony.
Origin
The face's first known appearance is tied to the comic "Nobody is Right," posted on DeviantArt on September 16, 2008 by the user 3-Angled-Blue. As early as March 2009 a similar character appeared in the comic "Troll Bait," created by a user named Strongside on the Facepunch Studios forum.
The version known as Sweet Jesus appeared in February 2011: a Redditor named Volco posted a comic on Reddit titled "Sweet Jesus Have Mercy!" In it the smile is flipped and the pupil of the left eye is shifted toward the edge, which produced the new variant of the face. The comic itself had multiple panels and showed a hungry protagonist finally getting his pizza delivery.
Spread
After the Reddit post, interest in the face spiked sharply: searches for "sweet jesus face" and "sweet jesus have mercy" shot up in March 2011, about a month after the post. The face quickly became one of the popular reactions users exchanged in comments.
The face was often compared to Me Gusta, another rage character with a similar emotional range. The Reddit community debated which one was appropriate in which context: the consensus was that Me Gusta suited something outright strange or dirty, while Sweet Jesus suited a more general sense of intense pleasure.
Typical captions and usage
The face is used to express intense longing or pleasure: from anticipating food or rest to delight over an unexpected gift. The caption is usually short and describes the situation, with the face itself serving as an emotional comment.
Unlike many other faces, what matters here is not irritation but the opposite feeling. That makes it a handy contrast to rage characters, and it's often placed next to images of a character suffering, creating an ironic switcheroo effect.
Why the face stuck
The face's main strength is hyperbole. The emotion is pushed to the limit: the face seems to have lost control from sheer bliss, and the viewer sees in it an exaggeration of an ordinary feeling. This caricature quality makes the reaction recognizable even without a caption.
The simplicity of the drawing also played an important role. The face was easy to copy or redraw, and users created their own variations while keeping the recognizable shape of the mouth and eyes. That let the face outlast many of the genre's one-day characters.
Legacy
Today Sweet Jesus Face shows up more often as a reaction image than in full comics. It has survived in the language of internet humor as an example of how a rage character can outgrow a specific genre and become a standalone emotion.
Sources don't give an exact point at which the face stopped being mainstream. But its history shows clearly how fast internet formats changed: from an MS Paint drawing on DeviantArt to a reaction understood by hundreds of thousands of people.