I don't think it's possible to prove with 100% certainty what Lavrov is thinking (assuming he is capable of forming thoughts), but he would have to be one of the dumbest people on the planet to believe a lot of what he himself says.
Here's an example from 2016:
>>>“If you’re talking about the Budapest Memorandum, we have not violated it,” he told a journalist. “It contains only one obligation — not to use nuclear weapons against Ukraine.” . . . Keep in mind that Lavrov wasn’t just Russia’s ambassador to the UN back in 1994; he actually signed the original Budapest Memorandum. He hasn’t been foreign minister for 12 years because he’s an idiot. He was lying, he knew he was lying and he knew everyone knew he was lying — and he didn’t care.
In the words of American diplomat Steven Pifer, who helped negotiate the Budapest Memorandum, “what does it say about the mendacity of Russian diplomacy and its contempt for international opinion when the foreign minister says something that can be proven wrong with less than 30 seconds of Google fact-checking?”
The answer: it says that, when it comes to the Kremlin, we’re dealing with a way of lying we’re not used to seeing. It’s a black-knight-from-Monty-Python-esque style of lying that makes dealing with Putin and company on issues like Ukraine and Syria such a nightmare.<<<
Lavrov was lying, he knew he was lying and he knew everyone knew he was lying — and he didn’t care.
nationalpost.com
Here's a more recent Lavrov whopper from March 10, 2022, after Russia attacked Ukraine on February 24, 2022:
“We are not planning to attack other countries. We didn’t attack Ukraine in the first place”
Speaking in Turkey after peace talks with Ukraine’s foreign minister Dmytro Kuleba failed, Lavrov repeated his previous assertions that Russian forces never invaded its neighbor.
nypost.com
I suppose it's theoretically possible that Lavrov is so spectacularly dumb that he believes what he's saying, but it is beyond my imagination.