A while back I talked to a fellow Lutheran priest and and we talked, amongst other things, about some traditional differences between Lutheran and Roman Catholic views on our relation to Christ, and about the difference between imitatio Christi, which is a traditional emphasis in Roman Catholic theology, with an emphasis on our imitation of Christ (cf. Philippians 2:5, 1. Peter 2:18-25), and conformitas Christi, which is a traditional emphasis in Lutheran theology, and particularily in Luther’s own though, with an emphasis on our conformity to Christ (cf. Romans 8:28-30). I believe that those two concepts are both very important, but I also believe that something is lost when we see them in a kind of duality.
I do agree with most Lutheran theologians that conformitas Christi is ‘more important’ than imitatio Christi, where the second follows from, or flows forth from, the first, but I don’t think that we will really grasp them until we stop viewing them dually, and start to see both, in their proper relation, as following from, and being based on, the more basic notion of participatio Christi. Participatio Christi is often seen as an aspect of conformitas Christi, but I don’t agree with that. We partake of Christ, logically speaking, before we are conformed to him, conformed to his image. The former is given us directly, through faith, in baptism, where Christ is truly present in the believer and the believer truly partakes of him, and the latter is a process through which God ‘molds’ us; forms us in, or conforms us to, the image of his Son (Romans 8:28-30).
In fact, I believe that this notion is at the heart of theology, and it is one of the main elements of my Lutheran defence of the Eucharistic sacrifice. When we properly understand our relation to Christ, through the hypostatic union, and expressed in (the Lutheran understandig of) the doctrine of the communicatio idiomatum, we see that it all boils down to this: Undeservedly, by grace, we are justified and made children of God, partakers of Christ, which, through the working of God, conforms us more and more to Christ, and which, again through the working of God, produces in us an imitation of Christ or what Christ and St. Paul calls ‘fruit.’ Note the important part of that image. No tree can force fruit to come. If the tree is good, and if it is well ‘fed,’ it will produce fruit. And we cannot produce fruit, says Christ, unless we are in him (John 15). He is the true vine, we are the branches, having been grafted into him.
When people argue what is more important; conformity to Christ or imitation of him, I say that they are both crucially important but must be understood in their proper relation to each other and, more importantly, to the more basic notion of our participation on him. Without that as the starting point, it all collapses and we end up emphasising ourselves (either inwardly or outwardly) instead of Him.
 For some points about this read Per Lønning, «Conformitas Christi,» in Lønning, The Dilemma of Contemporary Theology: Prefigured in Luther, Pascal, Kierkegaard, Nietzsche (Oslo: Universitetsforlaget 1962): 9-26, and Bård Norheim, Practicing Baptism: Christian Practices and the Presence of Christ (Eugene, OR: Pickwick Publications 2014): 104-106, 160-162, 174-176.
 For some ideas on this, see Union with Christ: The New Finnish Interpretation of Luther, eds., Carl E. Braaten & Robert W. Jenson (Grand Rapids, MI: Eerdmans 1998).
 See «“Do this in remembrance of me … ” A Lutheran defense of the Sacrifice of the Mass,» which is the accepted manuscript of an article of mine published by Taylor & Francis in Studia Theologica: Nordic Journal of Theology on May 8, 2017.
 See esp. Johann Anselm Steiger, «The communicatio idiomatum as the Axle and Motor of Luther’s Theology» (Lutheran Quarterly 14, 2000): 125-158 and Vidar Haanes, «Christological Themes in Luther’s Theology» (Studia Theologica 61, 2007): 21-46 (esp. pp.30-33).