Nope, no magic belief here. Hard to find on a skeptic's board.
On the subject of the lapse in development in Europe, let's not forget the catastrophic demise of a staggering percentage--somewhere between a fifth and a third, depending on whose numbers you accept--of the population due to the Plague (which we now think came from Asia) in the 14th century. Not only was the widespread death--and famine caused by the lack of labor to farm and transport food--a direct cause of society slumping to barely more than survival; it also caused people to be hysterically afraid of anything new or different, lest it bring pestilence with it. The Plague was not a single wave, but a series of epidemics, such that it must have seemed that life would always be a brief term of health between widespread and horrible death.
Without that problem, the fall of Rome might well have led to some smaller, more cohesive statelets that would have been able to push knowledge forward. But learning anything beyond the most practical of skills is a pastime of the wealthy, who can afford leisure. With the constant threat of disease and starvation, who could or would spend time or effort on something as impractical as trying to preserve or understand the knowledge of the ancients?
Oh, yeah: Some orders of the Church, actually. Latin language, and in many areas literacy per se was maintained by the needs and influence of the Church. While popular imagination thinks of the medieval Church as burning books, that kind of silliness was much more prevalent in the later, more wealthy and leisured Renaissance period (Savonarella, for instance).
I'm with Arthwollipot: The poster referenced in the OP is a very cheap joke. The failings and limitations of Christianity are many, but blaming it for the interregnum between Rome and the Renaissance is neither fair nor reasonable.
Just my thoughts, MK
ETA: We aren't exploring Space because it's a prohibitively expensive task, both in terms of money and of energy. Sending out some unmanned probes is probably all we will be able to do until and unless some remarkable (and now unforeseen) source of power is discovered. Even with fusion engines and substantial fraction of C -speed travel, a small ship with few people or resources onboard will take years to get to even the nearest star. Space is BIG. The challenges of trying to plan for /guesstimate everything that can go wrong on a years-long journey when hitting something the size of a pebble can destroy the mission* are considerable. That's why we aren't exploring the galaxy.
* Remember, kinetic energy increases by the SQUARE of velocity. The faster the ship is travelling, the more a dust-speck can destroy.