The nature of predestination, then, should now be clear. In general it is the divine assigning of each thing to its own purpose, but the predestination of the saints, which is the apostle’s subject here, is the election and destining of the saints for eternal salvation.
Now to the second part of our inquiry: why should we consider predestination? The teaching of Philip Melanchthon answers this very devoutly and faithfully: it is solely in order that you may be more certain of your salvation and may cleave more firmly to the promises of God.20 The first demand God makes of us is to believe that he is God, that is, the Saviour, so that when we hear him summoning to himself all who are afflicted and distressed21 we hasten eagerly to him. Now if those whom God calls heed his call, he has assuredly predestined and foreknown them, and will also justify and glorify them. Therefore, the first duty you owe to God is to believe that you have been predestined by him, because unless you believe that, you represent him as making sport of you when he calls you to salvation through the gospel. For by the gospel he summons you to justification and to share his glory, but these can be experienced only by those who have been predestined, foreknown and elected to do so. All God’s works are wrought in wisdom, and therefore by predetermined design. Consequently, if you doubt that you are predestined, you are also bound to doubt that you have been called to salvation, that you are justified, and that finally you are to be glorified; which means and to doubt the gospel itself, that is to say, to believe God for nothing at all of what he offers you in the gospel. For it is true believers who have eternal life, and they can no more doubt that they have it than doubt the Lord’s promise that ‘He who believes in me has eternal life.’22
From these comments, then, it will be quite evident that the reason why we and others should reflect upon God’s predestination is so that our faith in the promise of God may be strengthened by the knowledge that, as the apostle here affirms, the saints may have complete certainty that those whom God has predestined he will also call, justify and glorify. and those whom he has already called he has also undoubtedly foreknown and predetermined. This is the purpose behind every mention by the apostle of election and predestination, wherever it occurs. If we ponder this carefully with regard to ourselves, our confidence in God will be increased and with it our love for him and for every good thing, just as, when we entertain doubt as to our predestination, along with disregard and enmity for God all kinds of evil find an entrance. Therefore, we must reject as the source of every damaging temptation the question ‘Are we predestined?’ For as we have said, the person who is doubtful on this score will be unable to believe that he has been called and justified, which means that he cannot be a Christian. We must confidently trust, therefore, as the foundation of faith, that we have all been foreknown, predetermined, separated from the rest and chosen by God to this end, that we may enjoy eternal salvation, and this is God’s immutable purpose. Hence we must direct our whole attention and concern  to our response to this predestination and calling of God, so that we may work together with God unto eternal life, according to the strength which the Lord has ever supplied and for whose increase we ought unceasingly to pray.
But perhaps you are wondering about words such as “Many are called but few chosen’, and ‘Not everyone who says to me ‘Lord, Lord” shall enter the kingdom of heaven, but he who does my will.23 The former saying, ‘Many are called but few chosen’, refers to the fact that many are called through the gospel who nevertheless do not come to Christ, because they are not elect. As for yourself, therefore, believe the gospel and you have come!24 Then henceforth there will be no possibility of doubt in your case that you are foreknown, called according to his purpose, predestined and elect. The other saying, however, ‘Not everyone who says to me “Lord. Lord,”’ is meant to stimulate you to a keener zeal for the will of God, for that is why the Lord said it, not in order to engender doubts about your calling and election. Indeed, it is true that the greater your zeal for righteousness, the stronger will be your own and others’ certainty as to your calling and election25 for an earnest zeal for righteousness is so signal a gift of God as to be granted only to the predestined and elect. Thus a serious devotion to righteousness is complete proof that a man is predestined and elect. But because all our righteousness is so maimed and mutilated that by its merit alone we could never be assured of our salvation (for it fails to satisfy the law of God). assurance of our predestination and election is to be sought only from the promise and calling of God, and our attention must always be directed away from our own righteousness, which of itself is permanently an abomination in the sight of God, towards the divine promise. For the verse in the second Epistle ascribed to St. Peter–’Moreover. brethren, ‘be more zealous to confirm your calling and election, for if you do this you will never fall’–is not to be understood as if the stability of our election and calling depended to any degree on our works. Rather, by being zealous for good works we fulfil the purpose for which we have been called and elected, or, better still, we devote ourselves to working together with the Lord for its accomplishment. By ‘election and calling’ here is meant the life to which we are chosen and called, and so the subsequent words, ‘If you do this you will never fall’, amount to this: ‘From the way of the Lord which is your calling and election you will never fall away’, though of course we must guard against it by the earnest pursuit of godliness with the help of the Lord.
Martin Bucer, The Common Places of Martin Bucer, (England: Sutton Courtenay Press, 1972), 99-100. [Footnote values original; Underlining mine.]
20 Cf. Melanchthon, Loci Communes, 1535, sect. ‘On Predestination’ (CR 21,451 ff.).
21 Cf. Matt. 11. 28.
22 John 3. 15.
23 Matt. 22. 14, 7. 21.
24 Tu ergo crede Evmgelio et venisti. Cf. Augustine, credendo venisti; crede et venis; crede et manducasti, etc. (Sermon 30. 10, CCL 41, 389; Sermon 131. 2, PL 38, 730; Homilies on John’s Gospel 25. 12, CCL 36,254). Similar formulae are frequent in Luther
25 Cf. 2 Pet. 1. 10.