"Wherefore the rather, brethren, give diligence to make your calling and election sure: for if ye do these things, ye shall never fall." (2 Peter 1:10)
Although the calling of God is solely by His grace, apart from works (2 Timothy 1:9), and although "the gifts and calling of God are without repentance" (Romans 11:29), it is quite possible for a person to believe mistakenly that he has been called, and so Peter urges each professing Christian to make sure of his calling.
In the first place, one who is truly called will love God (Romans 8:28), and such love should not be superficial, but with the whole heart and soul and mind (Matthew 22:37). One who is called should "walk worthy of the vocation wherewith ye are called, With all lowliness and meekness, with longsuffering, forbearing one another in love" (Ephesians 4:1-2). If our lives fail such tests, we should at least "examine yourselves, whether ye be in the faith; prove your own selves" (2 Corinthians 13:5).
God's call is not only unto salvation, of course, but to a particular service for His glory. Some like Paul were "called to be an apostle, separated unto the gospel of God" (Romans 1:1). Many in Paul's day were actually called to be slaves: "Let every man abide in the same calling wherein he was called. Art thou called being a servant? care not for it: but if thou mayst be made free, use it rather. For he that is called to the Lord, being a servant, is the Lord's freeman: likewise also he that is called, being free, is Christ's servant. . . . Brethren, let every man, wherein he is called, therein abide with God" (1 Corinthians 7:20-22, 24).
Even the apostle Paul, however, could still say (and so should we), "I press toward the mark for the prize of the high calling of God in Christ Jesus" (Philippians 3:14). We need, first of all, to make our "calling and election sure." HMM