Very well stated, Etienne; and much more succinctly than I could have done. Apparently we have a mini-brouhaha over the sensitivities of some here to the use of the word atheist. As an Orthodox I have no problem with the use of this adjective in the context of the Sigillon of 1583. Simply put, adjusting the Church to fit 'observed' phenomena in the physical world is, by definition, outside (or denying), spiritual Truth.
It's about time you cracvked a dictionary yourself here; you are throwing words around platitudinously and inaccurately.
It's an unremarkable but necessary property of the truth that it be true.
Let's take that word "atheist". If one can see in the Greek text that it was not meant to signify the English meaning of the word, then it's simply a bad translation. (Mind you, Orthodoxy is not a legitimate qualification to be an astronomer, but that's a later point.) But if it was intended to signify the English meaning, then it's not only untrue: it's a malicious lie.
Let's rewind this a bit. Passover is supposed
to work from observed
phenomena. It says that in the Torah; this is not a negotiable point. The intent of Nicea is to put Easter on the Sunday after Passover; therefore Easter should be the first Sunday after the first full moon after the vernal equinox.
For various practical
reasons, we are now working off of mechanical calculations instead of actual observation. If we let the astronomers run things, they would be announcing each year whether or not there would be a leap day so as to fix the equinoxes on the same day. Ancient communications simply didn't permit such a method.
The Gregorian calculation isn't perfect; it does allow the actual equinox to move back and forth by a day, and there is a infinitesimally increasing error which some day in the quite distant future will require a correction.
An analogy might be appropriate- the adjective moral is opposed by amoral, not by immoral. Amoral denotes the lack of morals. So to would Atheist denote a lack of Godliness or something outside of the Church.
That's nice, but that's also not the way English works.
Given that ALL that is outside of the Church is meaningless to a Christian, I see no problem with the wording so used.
Speaking of platitudinous-- this statement is so far over the top that I don't for a moment believe that you actually live this way.