Were the Old Testament heroes like Jacob, Isaac, and David saved by faith alone?
No, they were not saved by faith alone. Jacob, Isaac, and David were men of faith, but their faith was joined with temple worship, law keeping, and other human works. Abraham, however, simply believed God, and righteousness was imputed to him without doing anything; the same is true for us.
Les explains from Hebrews 11 that Abel and Noah acted on faith: Abel offered a more excellent sacrifice, and Noah built an ark. Their faith was faith plus works. He then says all the way through Scripture, all those great men of God—Jacob, Isaac, and David—were men of faith, but not faith alone; their faith was accompanied by temple worship, law keeping, and other additions. By contrast, Abraham offered no sacrifice until years later. He simply believed God, and God imputed righteousness to him. Les applies this to us: without doing a thing, we believe Christ died for us and was raised in resurrection power, and that is what God looks for.