What does the stone that smote the image becoming a great mountain mean?
The stone that smote the image represents the returning Christ. Its becoming a great mountain that fills the whole earth signifies that He establishes His Kingdom, replacing the Gentile empires. This Kingdom fills the earth after the residue of those empires is blown away like chaff at His Second Coming.
Les explains that in Daniel 2:35, the stone that smote the image is the returning Christ. After Christ crushes the residue of the Gentile empires and destroys the Antichrist's armies at Armageddon, those empires become like chaff blown away by the wind. The stone then becomes a great mountain and fills the whole earth, meaning He sets up His Kingdom. Les notes that before bringing in the Jewish resurrected believers and fulfilling Israel's Old Testament promises, there are 75 days used to prepare for this Kingdom economy and cleanse the earth of all residue.
Then was the iron, the clay, the brass, the silver, and the gold, broken to pieces together, and became like the chaff of the summer threshingfloors; and the wind carried them away, that no place was found for them: and the stone that smote the image became a great mountain, and filled the whole earth.