Einstein's 1905 first postulate (the principle of relativity) is obviously true while the second postulate (the constancy of the speed of light) is obviously false. However, if the world can be misled into believing that the constancy of the speed of light is a logical consequence of the principle of relativity, or, equivalently, that variable speed of light contradicts the principle of relativity, the problem with the obvious falsehood is fixed. The consequence, the constancy of the speed of light, becomes just as true as the premise, the principle of relativity.

Einstein (and later Feynman) did manage to mislead the gullible world:

Albert Einstein: "If a ray of light be sent along the embankment, we see from the above that the tip of the ray will be transmitted with the velocity c relative to the embankment. Now let us suppose that our railway carriage is again travelling along the railway lines with the velocity v, and that its direction is the same as that of the ray of light, but its velocity of course much less. Let us inquire about the velocity of propagation of the ray of light relative to the carriage. It is obvious that we can here apply the consideration of the previous section, since the ray of light plays the part of the man walking along relatively to the carriage. The velocity W of the man relative to the embankment is here replaced by the velocity of light relative to the embankment. w is the required velocity of light with respect to the carriage, and we have w = c - v. The velocity of propagation of a ray of light relative to the carriage thus comes out smaller than c. But this result comes into conflict with the principle of relativity set forth in Section V." http://www.bartleby.com/173/7.html

Albert Einstein, On the Principle of Relativity: "After all, when a beam of light travels with a stated velocity relative to one observer, then - so it seems - a second observer who is himself traveling in the direction of the propagation of the light beam should find the light beam propagating at a lesser velocity than the first observer does. If this were really true, then the law of light propagation in vacuum would not be the same for two observers who are in relative, uniform motion to each other - in contradiction to the principle of relativity stated above." https://einsteinpapers.press.princet.../vol6-trans/16

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