It is common in Reformed Dogmatics to say that we have been chosen in order that we might be united to Christ. In that sense, union with Christ is the goal of election. Louis Berkhof says explicitly that, "Christ as Mediator is not the impelling, moving, or meritorious cause of election". Christ is merely the "mediate cause of the realisation of election." (Systematic Theology, Banner of Truth Trust, 1984, p. 114). But the New Testament seems to imply that we are chosen in Christ, not simply chosen that we might be united to him. The elective decision of God is itself Christocentric. Consider these two Scriptures:
"Blessed be the God and Father of our Lord Jesus Christ, who has blessed us with every spiritual blessing in the heavenly places in Christ, just as He chose us in Him before the foundation of the world, that we should be holy and without blame before Him in love" (Ephesians 1:3 & 4)
"Therefore do not be ashamed of the testimony of our Lord, nor of me His prisoner, but share with me in the sufferings for the gospel according to the power of God, who has saved us and called us with a holy calling, not according to our works, but according to His own purpose and grace which was given to us in Christ Jesus before time began" (2 Timothy 1:8 & 9).
Robert Letham points out that Reformed Theology was comfortable with the idea of the people of God being elected in Christ until the advent of the Arminian controversy in the 17th century. Jacobus Arminius taught that God elected those whom he foresaw would believe in Christ. Talk of Christ as the basis of election therefore sounded suspiciously Arminian to sensitive Calvinistic ears. Indeed, when Martinus of Breme proposed Christ as the foundation of election during the famous Synod of Dort, Gomarus, a conservative Calvinist challenged him to a duel! (See Robert Letham, The Work of Christ, IVP, 1996, p. 55).
The separation of Christ from election in later Reformed Theology is to be regretted. It has sometimes led to Christians having trouble with assurance of salvation. How can we know that we are elect if election is considered apart from Christ? Calvin had a much more Biblical and pastorally helpful account of the role of Christ in election. He establishes that we are chosen not in ourselves, with the goal of being united with Christ, but we are loved and elected in him,
"First, if we seek for the paternal mercy and favor of God, we must turn our eyes to Christ, in whom alone the Father is well pleased (Mt. 3:17). When we seek for salvation, life, and a blessed immortality, to him also must we retake ourselves, since he alone is the fountain of life and the anchor of salvation, and the heir of the kingdom of heaven. Then what is the end of election, but just that, being adopted as sons by the heavenly Father, we may by his favor obtain salvation and immortality? How much soever you may speculate and discuss you will perceive that in its ultimate object it goes no farther. Hence, those whom God has adopted as sons, he is said to have elected, not in themselves, but in Christ Jesus (Eph. 1:4); because he could love them only in him, and only as being previously made partakers with him, honor them with the inheritance of his kingdom".
It is foolish, harmful and dangerous to contemplate election apart from Christ. He is the mirror of election. We know that we are elect by believing in him and contemplating him,
"But if we are elected in him, we cannot find the certainty of our election in ourselves; and not even in God the Father, if we look at him apart from the Son. Christ, then, is the mirror in which we ought, and in which, without deception, we may contemplate our election. For since it is into his body that the Father has decreed to ingraft those whom from eternity he wished to be his, that he may regard as sons all whom he acknowledges to be his members, if we are in communion with Christ, we have proof sufficiently clear and strong that we are written in the Book of Life". The Institutes of the Christian Religion III:24:5 (here).
John Murray's exegetical approach to systematic theology alerted him to this aspect of election. According to him, "The people of God are not contemplated even in the purpose of grace apart from Christ (Ephesians 1:4)." (Redemption Accomplished and Applied Banner of Truth Trust, 1979, p. 93). He expands on this later in the book when commenting on the text to which he just referred,
"The Father elected from eternity, but he elected in Christ. We are not able to understand all that is involved, but the fact is plain enough that there was no election of the Father in eternity apart from Christ. And that means that those who will be saved were not even contemplated by the Father in the ultimate counsel of his predestinating love apart from Christ. As far back as we can go in tracing salvation to its fountain we find "union with Christ"; it is not something tacked on; it is there from the outset." (Op cit. p. 162).
If we would make our callling and election sure, we must not look to the elective decree in itself, but to Christ, the mirror of election.