There are certainly important and marked differences between Lutherans and the Reformed traditions, e.g., the rule of worship, Christology, the Supper, Baptism, church government, perseverance of the saints, covenant theology, and reprobation to name a few but it seems to be more an a priori assumption rather than a matter of fact that the confessional Lutheran and Reformed traditions are fundamentally divided on this point. So, the answer to the question is no and yes. No, it is not a distinctively Lutheran doctrine to say that the elect are mystically united to Christ, by the Spirit, through faith. 

I regularly get this question or its variant (is it semi-Pelagian to say that we are mystically united to Christ through faith?). I have answered the latter here. It might help the discussion if the reader consults the prior essay before continuing with this one since this one assumes the prior.

It seems fairly well established that the historic Reformed view of union with Christ is that there are three aspects of union: decretal, federal, and mystical. The last of these three aspects has been controversial in some confessional Reformed circles for 25 or 30 years. The traditional Reformed view was 1) there is an ordo salutis (this is in doubt in some Reformed circles now), i.e., a logical order in the doctrine of salvation. 2) That order is this: only the elect are regenerated (given new spiritual life), the regenerated are given faith with its attendant benefits (e.g., justification, adoption), and it is through faith that the Holy Spirit mystically unites the elect to Christ.

This is the teaching of the Heidelberg Catechism (1563). Question 32 asks “But why are you called a Christian?” It answers in part: “Because by faith I am a member of Christ and thus a partaker of His anointing…”. Please notice the instrumental language. “By faith I am a member of Christ…”. Faith is the instrument not only of our justification and adoption but also of our union with Christ. This is the teaching of Heidelberg 65. The premise of the question is that we are united to Christ, by the Spirit, through faith. It asks, “Since then we are made partakers of Christ and all His benefits by faith only, from where comes this faith?” The answer is particularly important here since it reaffirms our conviction that new life and true faith are worked in us and given to us by the sovereign prevenient work of the Holy Spirit: “The Holy Spirit  works faith in our hearts¹ by the preaching of the Holy Gospel, and confirms it by the use of the Holy Sacraments.” In Belgic Confession (1561) article 22 we confess “the Holy Spirit kindles in our hearts a true faith that embraces Jesus Christ, with all his merits…”. The Reformed were wont to attribute to faith all sorts of benefits. The French Reformed churches confess “by this faith we are regenerated (sanctified) in newness of life, being by nature subject to sin, Now we receive by faith grace to live holily and in the fear of God…”

The Westminster Shorter Catechism is quite clear not only about the logical order of the application of redemption but also about the place of mystical union with Christ in that order. According to question 29, we are made “partakers of the redemption purchased by Christ…by the effectual application of it to us by his Holy Spirit.” It should surprise no one to learn that the Westminster Divines were Augustinians. Question and answer 30 are central to this debate:

Q. 30. How doth the Spirit apply to us the redemption purchased by Christ?

A. The Spirit applieth to us the redemption purchased by Christ, by working faith in us, and thereby uniting us to Christ in our effectual calling.

Please note that the Spirit uses an instrument to apply to us “the redemption purchased by Christ.” What is that instrument? It is faith. Notice the instrumental clause: “by working faith in us and thereby uniting us to Christ” (emphasis added). “Thereby” is an adverb. It signals “by that means” (so the Oxford American Dictionary). Judging by the historical examples given in the Oxford English Dictionary to illustrate its meaning over time, his is the very sense it had when the divines used it in the mid-17th century. Faith is the instrument whereby the Spirit mystically unites his elect to Christ. In other words, it is not the case that we are mystically united to Christ in regeneration and that faith and works flow out of that union but rather, the Spirit uses faith to unite the regenerate to Christ and out of that (as we saw above) sanctification flows. The “thereby” of this question and answer repeats the clause “by working faith in us.”

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