Therefore, brothers, be all the more diligent to confirm your calling and election, for if you practice these qualities you will never fall (2 Pet 1:10).
i) This is sometimes thought to pose a problem for Calvinism. If Calvinism is true, how can we do anything to make divine election more certain than it already is? Is it possible to drop out of the elect? A few brief clarifications:
ii) There's a danger of overinterpreting Peter's usage. We need to distinguish between technical usage and ordinary usage. When we encounter words like "election" and "calling," there's a risk of reading later dogmatic usage back into Peter's vocabulary. But we can't assume Peter is using "election" and "calling" in the sense of God's eternal, unconditional election, or effectual calling. That may overload the usage with subsequent refinements in theological nomenclature.
His usage may not be that specialized. Indeed, he may be using "election" and "calling" as synonyms, for emphasis. The same definite article governs both nouns. So the pairing may be rhetorical.
iii) If his usage is nontechnical, then "calling" and "election" refer to God's initiative in salvation. We are saved by grace. It begins with God.
iv) In Reformed theology, although regeneration is the result of God's prior, unilateral action, once a person is born again, there's a sense in which he can "cooperate" in the process of sanctification. Sanctification involves the cultivation of godly attitudes. "Mortification of sin". Using the "means of grace". These are conscious actions on our part. Moreover, holiness is a matter of edge. Some Christians are more saintly than others. As A. A. Hodge and B. B. Warfield explain in their article on sanctification:
(1) The soul after regeneration continues dependent upon the constant gracious operations of the Holy Spirit, but is, through grace, able to cooperate with them.
(2) The sanctifying operations of the Spirit are supernatural, and yet effected in connection with and through the instrumentality of means: the means of sanctification being either internal, such as faith and the cooperation of the regenerated will with grace, or external, such as the word of God, sacraments, prayer, Christian fellowship, and the providential discipline of our heavenly Father.
(3) In this process the Spirit gradually completes the work of moral purification commenced in regeneration. The work has two sides: (a) the cleansing of the soul from sin and emancipation from its power, and (b) the development of the implanted principle of spiritual life and infused habits of grace, until the subject comes to the stature of perfect manhood in Christ. Its effect is spiritually and morally to transform the whole man, intellect, affections, and will, soul, and body.
(4) The work proceeds with various degrees of thoroughness during life, but is never consummated in absolute moral perfection until the subject passes into glory.
v) In Reformed theology, "cooperation" with sanctifying grace isn't "synergistic" in the libertarian sense. Our cooperation is, itself, the result of grace. Moreover, the outcome is assured.
vi) Salvation begins with God's initiative, but it doesn't end where it begins. Having revived those who are dead in sin, they can pursue the journey of faith until they arrive at their final destination. In that way we "validate" God's initial intervention.