How could Hushai remain loyal to David? Where did he learn this kind of loyalty?

Hushai could remain loyal because he had kept his eyes on the Word of God rather than on any man. He understood that our part is to make the Word the issue, give people the right to accept or reject it, and then let God deal with them—that it is not our job to convict or judge another believer, for those are rights God reserves to Himself. He learned this loyalty by watching David in the wilderness and the mountains, hounded by King Saul who was himself in spiritual retreat. He saw David trust the Lord and entrust himself to God even under an authority in reversionism, and from what he saw he became one of the most noble and honorable men of his generation.