Excellent questions! A few years ago I was riveted by II Peter 1:5-11. In verse 5, the Apostle commands us to "make EVERY effort" to add to our faith, to add to our goodness, to add to our knowledge, to add to our self-control, to add to our perseverance, to add to our godliness, to add to our brotherly kindness, and to add to our love; and then he tells us why this command is so crucial: "For if you possess these qualities in increasing measure, they will keep you from being ineffective and unproductive in your knowledge of our Lord Jesus Christ." (II Pe 1:8) Here is the very point you were making. Many of us profess faith in Christ, but often our lives are "ineffective and unproductive." The reason, according to this passage, is that we are not making any effort (or enough effort) to add anything to our faith, to our goodness, to our knowledge, to our self-control, to our perseverance, to our godliness, to our brotherly kindness, or to our love. Or perhaps we're content to add to our knowledge (it is so easy to close the door and study), but we're avoiding adding anything to our brotherly kindness or to our self-control.
The next verse is stunning: "But if anyone does not have them [these qualities in increasing measure], he is nearsighted and blind, and has forgotten that he has been cleansed from his past sin." Phew. Not simply lazy. Not simply idle. The messenger of Christ calls us "nearsighted and blind" when we fail (or refuse) to add to our walk. He says we have forgotten what Christ has done for us. That is a heavy conclusion.
But he does not stop there! Verse 10 has a promise that is too good to believe (but Scripture is true, so we can (and must) believe it!): "For if you do these things, you will NEVER FALL..." Never fall? Never?
No wonder the writer of Hebrews exhorts us to RUN with perseverance the race set before us (He 12:1)! We learn from this passage in II Peter that the very running of the race produces fruitfulness and protects us from falling. No wonder David writes joyfully, "I RUN in the path of your commands, for you have set my heart free." (Psalm 119:32)