It is true that the work of Christ is a finished work in the sense that there is nothing we can do or add to it, it is equally true that we must actively participate in our salvation. As Paul states in Philippians 2:12, we are called to work out our salvation with fear and trembling. Faith is an active faith, and salvation in its true sense is a life to be lived out in the power of the Holy Spirit (Gal 2:20, Romans 8:13-14).
Diligence means to give wholehearted effort to something. A diligent person is one who takes great care, goes to great length, and perseveres against all odds, to accomplish a purpose. They accomplish what they set out to do, because it is of utmost importance to them.
Contrary to those who claim that this amounts to mere works done in a legalistic spirit, the Bible sets forth plainly that we are to be diligent in our Christianity. There are two passages in 2 Peter that we will look at.
Grace and peace be multiplied to you, in the knowledge of God and of Jesus our Lord; seeing that His divine power has granted to us everything pertaining to life and godliness, through the true knowledge of Him who called us by His own glory and excellence. For by these He has granted to us His precious and magnificent promises, so that by them you may become partakers of the divine nature, having escaped the corruption that is in the world by lust. Now for this very reason also, applying all diligence, in your faith supply moral excellence, and in your moral excellence, knowledge, and in your knowledge, self-control, and in your self-control, perseverance, and in your perseverance, godliness, and in your godliness, brotherly kindness, and in your brotherly kindness, love. For if these qualities are yours and are increasing, they render you neither useless nor unfruitful in the true knowledge of our Lord Jesus Christ. For he who lacks these qualities is blind or shortsighted, having forgotten his purification from his former sins. Therefore, brethren, be all the more diligent to make certain about His calling and choosing you; for as long as you practice these things, you will never stumble; for in this way the entrance into the eternal kingdom of our Lord and Savior Jesus Christ will be abundantly supplied to you. (2 Peter 1:2-11 NASB)This is working out our salvation with fear and trembling! We are not working for our salvation as if it were something we could earn or accomplish on our own, but working out what God is working in our lives (Phil 2:12-13). We have God given faith, we have God's promises to us as found in His word, and we are made partakers of His divine nature. We now must yield to Him, submit to the Lordship of Jesus Christ in obedience to His word. We, like Paul the Apostle, are to press on to maturity (Phil 3:7-17). The Christian life is a life of continual growth. It is impossible to remain stationary, and yet be alive as a Christian. We must be diligent in this or we risk drifting away from the hope of our salvation (Hebrews 2:1-3). This passages ends with the exhortation to make our calling and election sure. Again, this is not self effort, not a call to salvation by works, but an admonition to persevere in our life in Christ. If we are not following Jesus, if we are not walking in the Spirit, we are falling away from Christ, and risk our very souls. Salvation is a radical transformation of heart and life. It must be lived out in our daily lives if it is real.
The Apostle peter, in speaking of the return of Christ and the end of the age, has this to say,
But according to His promise we are looking for new heavens and a new earth, in which righteousness dwells. Therefore, beloved, since you look for these things, be diligent to be found by Him in peace, spotless and blameless. (2 Peter 3:13-14 NASB)Those who are longing for the return of Christ are to be diligent in their conduct today! John admonishes us to abide in Him so that we will not be ashamed at His coming (1 John 2:28-29). This involves living righteously. Again he writes that those who are looking for the appearance of Christ will purify themselves just as Christ is pure. Once again, this is speaking of the purity of our heart and actions (1 John 3:3-6). We are told that no one who abides in Him sins, as He has come to take away sins. We must be ever so diligent in our daily lives, each and every moment, that we are not led away from Christ through complacency or allowing sin in our lives. This takes effort, diligence.
Watch over your heart with all diligence, for from it flow the springs of life. (Proverbs 4:23 NASB)