These two terms seems to be used interchangeably, but I'm sure they are very different.

Am I correct to assume that the Imperial Creed is the actual "enlightment" of humans to follow and worship the Emperor as a divine being (for example, to colonies that were lost by the warp storms before and brought back into the imperium) while the Imperial Cult is the actual "Church"?

Thanks in advance !

As far as I understand, Imperial Creed is the doctrine, Imperial Cult is the church that exercises it. The relation between the two is like that between Catholicism and the Catholic Church.

That makes sense, and the bringing of the word of the emperor to those colonies is just one task within that doctrine.

As far as I understand, Imperial Creed is the doctrine, Imperial Cult is the church that exercises it. The relation between the two is like that between Catholicism and the Catholic Church.

This is correct, as far as I'm concerned. The Imperial Cult is the cult that practices and teaches the Imperial Creed.

The Imperial Cult is the same as The Ecclesiarchy, also known as the Adeptus Ministorum, although there's an argument that the Imperial Cult is larger than The Ecclesiarchy - the Ecclesiarchy can be argued to be the formalised structures and authorities revolving around the faith, while the Imperial Cult encompasses slightly more.

For example, the Frateris Militia could be considered belonging to the Imperial Cult, but clearly don't belong to the Ecclesiarchy, formally speaking.

It's a matter of the definition of the words cult and creed.

From what I understand you have it correct.

I always view the imperial cults as lay priests and the ecclesiarchy as the official priests that regulates the lay priests. If a single cult gets out of line it is up to the ecclesiarchy to step in and make some "corrections". Which is actually needed considering Imperium is so vast that it is physically impossible for the ecclesiarchy to be every where. The imperial cults is a necessary tool to keep the faith in the empire.

According to Lexicanum, the Imperial Creed is the collection of the core tenets of faith, the Imperial Cult is the entirety of religious practice (including the myriad local observances and doctrinal changes), and the Ecclesiarchy is the organization overseeing it all.

I was pretty much right the first time, if we take the word "church" to mean not just the structure, but all of the practitioners as well.