Were the Old Testament heroes like Jacob, Isaac, and David justified by faith alone?
No. Jacob, Isaac, and David were men of faith, but not by faith alone; their faith was joined to human works such as temple worship and law keeping. Abraham, however, simply believed God, and God imputed righteousness to him without his doing anything.
Les explains from Hebrews 11 that Abel offered a sacrifice and Noah built an ark, showing faith was accompanied by action. He then says all those great men of God, Jacob and Isaac and David, were men of faith, but faith alone? No. Their faith was plus temple worship, plus law keeping, plus this, plus that. Abraham, however, never offered a sacrifice until years later; he did not do anything, he simply believed God, and God imputed righteousness to him. Les applies the same pattern to us: without doing a thing, we simply believe that Christ died for us and was raised in resurrection power, and that is what God is looking for.