Why does it mention preparing mountains with His strength?
Rashi: He makes them sprout [even though] they are hard. He prepares for them food and rain - "ha'Mechin la'Aretz Matar ha'Matzmi'ach Harim" (147:8).
Radak: If the nations will say, how could Yisrael, a poor and lowly nation, scattered and separated, leave Galus? Who took them out? We find that they will say "ha'Yukach mi'Gibor Malko'ach", and Hashem answers "Gam Shvi Gibor Yukach u'Malko'ach Aritz Yimalet v'Es Yerivech Anochi Ariv" (Yeshayah 49:24-25) - I am stronger than them! Also here, the one who made these Gevuros and wonders can take us out from them. Tall mountains are among Hashem's wonders. This is why they are written next to Kel - "k'Harerei Kel" (above, 36:7) - His strength is seen from them.
Malbim: Tzedek does so; it prepares mountains with its strength. The world stands on it.
Why does it say that Hashem is "Ne'ezar bi'Gevurah"?
Rashi: It is because it will mention [below] the Gevurah of rain.
Radak: He is always girded with strength. He does not acquire it from another when needed; rather, He shows it whenever He wants to.
Malbim: Tzedek does so; it prepares mountains via dew and rain and makes grass grow. It is girded with strength; its power is very great.
