Lithuanians weren't actually Lithuanians, they were Russians! Well, no, they weren't Lithuanians or Russians, they were in fact some other group of Slavs that conveniently allied with the Poles against the actual Lithuanians!
And in support of our views, we have simplified maps of linguistic distribution from several centuries after the time period we're talking about that, in addition to actual research into what actual people spoke in certain places at that time, incorporate a significant amount of guesswork and personal prejudice!
One very solid argument would be Statues of Lithuania, the principle legal document of the country, that wasn't even written in Latin or Lithuanian, but only in Old Belarussian (as opposed to Modern Belarussian, which is a completely made up language), which was a local dialect of Russian.

Spoiler :

And here is a list of soldiers of the Grand Duchy of Litva (Lithuanian army register) from 1528 (in Russian):
You can see that most of the surnames are of Ruthenian/Polish (Slavic) origin.
It's in Belarussian. Interesting. Most of those lastnames are still very common in Russia.
It's interesting how history worked out. The Samogitians gained a history they never truly had, and the Belorussians somehow lost the history they had. And Lukashenko is keen on ridding them of the last few symbols from the era like the Pahonia and the white, red, white Litvin flag.![]()
Halp! Jmud' be stealing our history.

In taberna quando sumus, Belarussian style:
Link to video.