In Russian, почва (pochva) is the soil, the ground in which something grows. In the prince's speech against Catholicism, the word appears in the maxim that whoever has no ground beneath them has no God. In the Russian text he says the maxim is not his own, but that of a merchant of the Old Believer faith whom he met on a journey, who phrased it by saying that whoever renounced their native land also renounced their God.
Where It Appears
Quotes
That is because he found the fatherland that he had failed to see here, and rejoiced; he found the shore, the earth, and threw himself to kiss it! [...] "Whoever has no ground beneath them also has no God".