For this very reason, make every effort to supplement your faith with virtue, and virtue with knowledge, and knowledge with self-control, and self-control with steadfastness, and steadfastness with godliness, and godliness with brotherly affection, and brotherly affection with love. For if these qualities are yours and are increasing, they keep you from being ineffective or unfruitful in the knowledge of our Lord Jesus Christ. For whoever lacks these qualities is so nearsighted that he is blind, having forgotten that he was cleansed from his former sins. Therefore, brothers, be all the more diligent to confirm your calling and election, for if you practice these qualities you will never fall. For in this way there will be richly provided for you an entrance into the eternal kingdom of our Lord and Savior Jesus Christ.
Do you want to know if you are a true believer?
Do you want a way to evaluate whether someone else is truly a believer?
Do you fear being an ineffective and unfruitful Christian?
According to these verses, those who have certain qualities, and are striving to increase them, are the ones who will make it to Heaven. By diligently seeking to have and increase these things, they manifest a confirmation that they are truly in the Way; and so doing will also keep them from falling, and it will make them effective and fruitful in their Christian walk.
Those who lack these qualities have forgotten that they were cleansed from former sins. They have drifted away from the truth; they have not paid all the closer attention to the gospel. They have neglected their salvation.
Hebrews 2:1,3
Therefore we must pay much closer attention to what we have heard, lest we drift away from it. … how shall we escape if we neglect such a great salvation?
Here are the qualities Peter mentions, in the order of the chain of connection:
Faith
Virtue / excellence
Knowledge
Self-control
Steadfastness / perseverance
Godliness
Brotherly kindness / brotherly affection
Love
Each one should result in the next one; to each one we are to add the one that follows.
Faith is the essential beginning of salvation.
To faith, add excellence; don't just believe and stay there - feed your faith, deepen it, have a robust faith.
Growing your faith will require saturating yourself with God's Word. So to your excellence of faith, add an increasing and deepening knowledge of God and His Word.
Knowing the Word gives us more insight into God's character and holiness, and more view of our own sin. This must lead to self-control, a striving to rule one's self by the Scriptures.
Self-control will enable perseverance and keep us from giving up when we wish, for we are ruling ourselves by Scripture and by the Spirit's help, rather than letting our flesh rule us.
Perseverance in the Way will bring us closer to God and make us more like Him (godliness).
When we become more like God, we will grow more out of ourselves and more focused on others, with brotherly kindness, especially for the brethren.
And brotherly kindness should lead to a God-like, self-sacrificing love for others.
If you are striving for these things, they will be evidence that you are Real; they will keep you from falling; they will cause you to be effective and to bear fruit for Christ; and they will bring you safely into Heaven at last. These things will be used by God to keep your soul and to make you persevere true to Him all the way to the end.
So if you fear being fruitless, or you wonder if you will stay on the Narrow Way, throw your energy into this chain. Begin somewhere. Feed your faith, that it might grow strong. Soak in the Scriptures, that you might know God, for it is this that will lead to more hatred of sin and more progress in virtue. Know God, and apply that knowledge to your spiritual walk; where do you have sins to deal seriously with, or areas of the flesh to gain self-control over?
Strive, to make your calling and election sure.
Evidence of salvation is found in those who are constantly striving to know more of God, to fight sin more strongly, to love more sacrificially. All for God's glory. All because Christ has cleansed me from my former sins.