First, you need to go read this blog at Storied Theology by Daniel Kirk. It's called "Giving up on Faith", but it's really just about faith.
Back now? OK. I have mentioned before that my dad considers a good working definition of faith to be "uniting life", so that when we become Christians, we are uniting our lives with Christ. And that really works as a definition.
I linked Kirk's article, because he brings out some of the same themes. Faith is more than just mental assent to some theological assertions. And that won't do. Believing that Jesus lived, died, and was resurrected isn't enough for salvation. And, if you believed that, why wouldn't your life change?
In actuality, Jesus makes the first move toward us by faithfully obeying God through his death, burial, and resurrection. Our job is to be faithful, or to unite our lives with Christ. But, Christ has already done the heavy lifting. Romans 1:5 says that "through him we receive the obedience that comes through faith."
Salvation and the abundant life here on earth are gifts to us by God's grace. We don't earn them. But, uniting our lives with Christ produces faithfulness and righteousness in our lives. They fit together. Grace and faith go together.