Tough one. “Ravir” (here) means steal, but in a subtle way, and often in the rules. For example in a race, if A is leading the whole race but B get in first place right at the end and wins the race, one could say “B lui a ravi la victoire“.
It can also be used for opinions and elections, (I believe that’s the case in your example) : like if voters are mostly voting for A and after a speech, B gains a lot of supporters from A, “ravir” could be used too. It implies “in the rules, but kind of sneaky”, and often “at the very last moment”
“Adhésion” means the support of other people. “Ils adhèrent à mes idées” : They agree with my ideas. It’s hard to tell with no context, but it could be political, or taking sides in a debate, or something like that.
So “ravir l’adhésion” means stealing or simply gaining the agreement and support of people who were before agreeing to someone else, most likely a competitor.
In your sentence, the author also used “intelligences” in lieu of “intelligent people”. So one person lost the support and agreement of a number of intelligent people, and is blaming it on popularity. The person speaking says they are wrong to do so.