How should the just live by faith?

"The just shall live by faith"

~ Habakukk 2:4

Words made famous by Martin Luther is a statement of righteous living. But what exactly does it mean to live by faith? There are many Hebrew words associated with faith but the one used here in the original Hebrew text is "emunah".

The link below has very clear explanations on the various Hebrew words.

http://internetbiblecollege.net/lessons/hebrew%20words%20for%20faith.htm

So emunah in its basic meaning is "certainty" and "faithfulness". So one can say "the just shall live by certainty and faithfulness (wrt God)".

I suspect many of us know about the certainty aspect of that verse, ie. trusting God, but lost the faithfulness portion which is important is order for faith to be there is the first place. Eg. We cannot say that we love our spouse if we trust them but yet go out philandering.