"Believed" is not a scientific term. Belief means having faith in something. It is not science. Science is observation first of all. It is verification of a theory, not belief. Now the question that should be asked is what is the basis for this belief? Is there anything scientific in it? Let's look up endosymbiosis:
The gap between eukaryotes, cells with nuclei, and prokaryotes, cells which lack nuclei, is considered by many biologists to be the most profound missing link in evolutionary history. In an attempt to describe the way in which this gap was bridged, scientists have proposed the serial endosymbiosis theory (SET). The term "endosymbiosis" specifies the relationship between organisms which live one within another (symbiont within host) in a mutually beneficial relationship. The SET states that the evolution of eukaryotes from prokaryotes involved the symbiotic union of several previously independent ancestors. According to the theory, these ancestors included a host cell, an ancestor of mitochondria, an ancestor of chloroplasts, and, more controversially, a prokaryote that brought with it the structures that today provide cellular motion.
FROM: Serial Endosymbiosis
Euglena, not only presents the problem of where did the eye come from, but also where did the cell nuclei come from and how a plant-animal arose. Note that nothing above, in your posts or in any of the articles linked to on this subject explains the eye of the euglena. Even the ridiculous "belief" in endosymbiosis does not provide an answer to that question. Let's also look at what is going on here. A new theory (endosymbiosis) is being created out of whole cloth to support another theory (evolution) which cannot explain how eukaryotes arose through descent from prokaryotes. It is pretty much like the Ptolemaic theory of geocentricity - as more was discovered, the more epicycles were created.
Shame you didn't read that article you linked for more than lawyering purposes. Organized religion has such a stultifying effect on some people, it's hard not to be poisoned against it.
New hyptotheses arise in science. Some of them are pretty interesting. Science makes progress. Religion doesn't. Religion should thus stay out of science.