5 ¶ And beside this, giving all diligence, add to your faith virtue; and to virtue knowledge; (revised)
Believing is one thing but faith must put on flesh to have a usefulness in God’s plan. As James told Jewish Christians in his letter, faith without works is dead.
James 2:26 For as the body without the spirit is dead, so faith without works is dead also.
Paul made it clear that we are called to something after we are saved.
Ephesians 2:10 For we are his workmanship, created in Christ Jesus unto good works, which God hath before ordained that we should walk in them.
The evidence or fruit of having God’s Spirit in us, being saved, is also made clear by Paul.
Galatians 5:22 But the fruit of the Spirit is love, joy, peace, longsuffering, gentleness, goodness, faith, 23 Meekness, temperance: against such there is no law. 24 And they that are Christ’s have crucified the flesh with the affections and lusts. 25 If we live in the Spirit, let us also walk in the Spirit.
Peter here begins building on the foundation of faith, given by God. He wants the Christian to do these things diligently, to cultivate these traits. First, adding to the faith given to us we seek virtue, the moral power of a changed life. Virtue is many things such as moral courage and the courage of your convictions, not standing on them for the purpose of forcing others to mirror you but the courage of living in the manner deliberately in which Christ has called you to live.
Romans 6:12 Let not sin therefore reign in your mortal body, that ye should obey it in the lusts thereof. 13 Neither yield ye your members as instruments of unrighteousness unto sin: but yield yourselves unto God, as those that are alive from the dead, and your members as instruments of righteousness unto God. 14 For sin shall not have dominion over you: for ye are not under the law, but under grace. 15 What then? shall we sin, because we are not under the law, but under grace? God forbid. 16 Know ye not, that to whom ye yield yourselves servants to obey, his servants ye are to whom ye obey; whether of sin unto death, or of obedience unto righteousness? 17 But God be thanked, that ye were the servants of sin, but ye have obeyed from the heart that form of doctrine which was delivered you. 18 Being then made free from sin, ye became the servants of righteousness.
This is very difficult for young people driven by biology and a need for validation and love, for certain sub-cultures within our country who regard sexual desire as a mere physical need to be satisfied, and for older people who have become obsessed with sex as a symbol of youth and health. But, moral virtue is fundamental to being a Christian in practice with fornication underscored here next in the context of adultery with another person’s spouse.
1Thessalonians 4:3 For this is the will of God, even your sanctification, that ye should abstain from fornication: 4 That every one of you should know how to possess his vessel in sanctification and honour; 5 Not in the lust of concupiscence, even as the Gentiles which know not God: 6 That no man go beyond and defraud his brother in any matter: because that the Lord is the avenger of all such, as we also have forewarned you and testified. 7 For God hath not called us unto uncleanness, but unto holiness. 8 He therefore that despiseth, despiseth not man, but God, who hath also given unto us his holy Spirit.
Knowledge here, because of the context, must be linked to the knowledge of Christ in verse 3. This was a desire of Paul which he expressed in 1Corinthians and Philippians.
1Corinthians 13:12 For now we see through a glass, darkly; but then face to face: now I know in part; but then shall I know even as also I am known.
Philippians 3:9 ¶ And be found in him, not having mine own righteousness, which is of the law, but that which is through the faith of Christ, the righteousness which is of God by faith: 10 That I may know him, and the power of his resurrection, and the fellowship of his sufferings, being made conformable unto his death; 11 If by any means I might attain unto [understand as in Psalm 139:6 and Proverbs 1:5] the resurrection of the dead. 12 Not as though I had already attained, either were already perfect [complete in understanding as in Col. 4:12 and 2Chronicles 8:16]: but I follow after, if that I may apprehend [to know, as in contrast to know in verse 10] that for which also I am apprehended of Christ Jesus. 13 Brethren, I count not myself to have apprehended: but this one thing I do, forgetting those things which are behind, and reaching forth unto those things which are before, 14 I press toward the mark for the prize of the high calling of God in Christ Jesus.
The knowledge of Christ is attained by prayerfully reading the Bible and submitting to Him and His authority over you and by exercising your faith in the practice of virtue which requires a conscious rejection of sin and its hold over your body and your mind.
Added to the faith which came from God is virtue or a desire to do right by God and then seeking the knowledge of Christ and His will for your life. How can you know Christ if you hold His standard for your life in utter contempt by your behavior?