It has often been said that T.F. Torrance’s “mechanism” (the means by which atonement is accomplished) of atonement is the vicarious humanity of Christ. However I’m starting to think that the vicarious humanity of Christ is just a part of a larger mechanism of atonement – which is actually union.
I recently came across a passage in Space, Time, and Resurrection 
where Torrance talks about the “mechanism” of atonement (although he doesn’t use that particular language. This passage is filled with vicarious humanity language – yet read a certain way, it seems as though even more foundational than vicarious humanity (this is actually what makes vicarious humanity possible) is actually the union between God and man. Here is how Torrance puts it:
By living the life which Jesus Christ lived in our midst, the life of compete obedience to the Father and of perfect communion with him, the life of absolute holiness in the midst of our sin and corruption, and by living through the whole course of our human existence from birth to death (this is vicarious humanity language) he achieved within our creaturely being the very union between God and man that constitutes the heart of atonement (this is union language) effecting man’s salvation and restoration to communion with God the Father.
Naturally we understand the fact that Christ can live our lives vicariously because human nature and the divine nature are hypostatically united in Jesus Christ – but it seems to me that Torrance is placing the emphasis not on the vicarious nature of Christ’s life on our behalf – but rather on the fact that union occurs (and persists) as the thing which constitutes atonement.
CWoznicki Think Out Loud 