The locus of spiritual formation and spiritual transformation is the human heart. I've given my RMS Spiritual Formation students the link to Dallas Willard's "Spiritual Formation: What It Is, and How It Is Done." We'll discuss this at the beginning of our class tomorrow morning.
Willard writes: "Spiritual formation in the tradition of Jesus Christ is the process of transformation of the inmost dimension of the human being, the heart, which is the same as the spirit or will. It is being formed (really, transformed) in such a way that its natural expression comes to be the deeds of Christ done in the power of Christ."
This means that one's heart is formed, or morphed, into Christlikeness (Gal. 4:19). Spiritual formation, Jesus-style, is not about asking "what would Jesus do?" and then trying hard to be like Jesus and do the things he did. If a person had the heart of Jesus the results would be, inexorably, the kind of interior and exterior life Jesus had. With the "interior" being ontologically prior to the "exterior." As Willard says, "Christlikeness is established in the very depths of our being." This is about "change of the inner person, where what we do originates."
(Willard's entire article is ridiculously profound, deep, and insightful. See also Willard's more recent "Spiritual Formation in Christ: A Perspective on What it is and How it Might be Done.")