Louis,
Thanks for the post. We ought to be in the Word consistently and often so as to "so that we may no longer be children, tossed to and fro by the waves and carried about by every wind of doctrine, by human cunning, by craftiness in deceitful schemes;" Ephesians 4:14.
Contextually, this is why we have pastors and teachers; Ephesians 4:11.
As we can see, humans play a major part in deceiving others with false doctrine. Even on forums we have those who are Judaizers, those with Hebrew Roots false doctrines, those maintaining salvation (so they teach) &c attempting, all of whom are attempting to impede our sole trust in Christ.
All cults share this same objective as well as all false teachers; the bottom line is they detract from Christ, detract and subtract his work, even though at the same time they say they are not doing this, it is exactly what they are teaching. Of course there are more cultic groups so no need to get exhaustive other than to point out the objective of all false teachers, that is, their "bottom line."
As far as Satan deceiving the whole world, yes I agree, yet, no, this does not mean each and every person who has lived, nor does it mean each and every person alive at that period. He will deceive the vast majority of persons in every nation across the globe. That is the meaning.
It appears impossible to deceive the elect; Matthew 24:24. So, that being said, contextually from your shared passage in Revelation, there will be some who are not deceived by him, that is, God's elect. He will always have and preserve his remnant.
God bless.