The flesh, its emotions and physical endurance, cannot accomplish God’s purpose for mankind. It is the gospel, according God’s own purpose and grace, that has saved us and called us to a holy life, not anything we have ourselves have done (2 Timothy 1:8-9).
Peter is an excellent example of God’s grace working in people who will by faith humble themselves and follow Christ. It is good for our hearts, like Peter’s, to be strengthened by grace (Hebrews 13:9). When we seek to be strengthened for our spiritual battle by our flesh and our own understanding, we deny the grace of God in Christ (Proverbs 3:5-6). We need not to deny God’s grace but to grow in it (2 Peter 3:18)!