What could be more honest than to recognize a good principle from repeated, repeatable personal experience? And that that is what an individual can know...

• ...know for themselves. Then, faith is to act on that principle, incorporating it into one's life and moving forward with it, without fail.
• And if the Spirit and our understanding concur (as they can with further faith+action), and it makes us better and happier, that is further confirmation.
• "Proof" of the gospel, as for anything else (works well for me, you have to decide for yourself): the relationship between science & religion, etc (main)