The 'analogy of faith' refers most fundamentally to a relation of correspondence between an act of God and an act of a human subject; the act of divine Self-revelation and the human act of faith in which that revelation is acknowledged. More specifically, the analogy which is established in a revelation event is an analogy between God's knowledge of Himself and human knowledge of Him in and through human concepts and words. There are three aspects of this analogy which need to be highlighted. First, the analogy in question is not posited with creation. It is not an analogy between the being of the Creator and the being of the creature---which Barth refers to as an analogia entis in contrast to an analogia fidei. The focus here is not being but rather a highly concrete event: the event of revelation. Second, there is nothing in the being or knowing of the human subject which helps to bring this event about---no capacity or pre-understanding which might be seen as a necessary precondition to its occurrence. The only capacity needed for the analogy is one which God Himself graciously provides in the event itself as a gift, namely faith. In the event of revelation, human knowledge is made by grace to conform to its divine object. Thus (the reader will forgive an overused metaphor, but it is good Barthian language), the direction in which the analogy works is always 'above to below'. That is to say, God's Self-knowledge does not become analogically related to prior human knowledge of Him in revelation; rather, human knowledge is conformed to His. God's act is the analogue, ours is the analogate; His the archetype, ours the ectype. Third, the 'analogy of faith' is to be understood 'actualistically', that is, strictly as an event. The relation of correspondence which is established in the revelation-event does not become a predicate of the human subject. To put it another way, the "being" of the human subject is not altered through the experience of faith's knowledge of revelation. The analogy endures only so long as the revelation-event endures. Thus, the 'analogy of faith', once realized, does not pass over into human control. It must continue to be effected moment by moment by the sovereign action of the divine freedom if it is to be effected at all. [Bruce L. McCormack, Karl Barth's Critically Realistic Dialectical Theology: Its Genesis and Development 1909~1936, (Barnes & Noble, Nook edition), 32]
I won't try to do an itemized reflection on all that McCormack develops for us, but let me notice something. A very important consequent of Barth's 'analogy of faith' is that God's Self-freedom remains just that. Creation (humanity) in its pursuit to know God is not allowed to construct a prior methodological apparatus that supposes a notion of God (simpliciter) prior to meeting God in Jesus Christ, at least not in Barth's accounting. Instead, the 'analogy of faith' requires that God both confronts and contradicts any conceptions of Godself that humanity has tried to construct on their own as a prius (no, not the Toyota ... silly ;)---this would be idolatry, and the kind of theology that springs from the so called 'analogy of being' (which McCormack touches upon in his development)---the kind of theology that has its high places, like what we see constantly in Israel's history (the syncretization of worshipping God through mixture and integration with human conceptions, using creation with our own hands to ultimately gain mastery and control over the god we [or they, Israel in the OT] worship).
So this is where Torrance appropriates his own special way of articulating the 'analogy of faith'; from his teacher, Karl Barth. It is one, obviously, that I advocate, and think that it is one that all genuinely Christian Dogmatic & Theological efforts should likewise adopt as their method for 'doing' theology, ethics, and life.
An aside: I am often accused of, apparently, pitting classic Calvinism against Evangelical Calvinism; as if I am artificially flavoring the waters to the point of making theological kool-aid out of this continuum (of Calvinism). So people often think that I am just naïvely creating a chasm between the two instantiations of Calvinism, either because I must be naïve or just plain antagonistic (both of which may be true, but NOT in this instance!). This issue right here (the topic of this post) is where the definitive distinction and wedge come between the two approaches. When I write classic Calvinism, I am not primarily using this designation to assert that this is the historic version of Calvinism V. Evangelical Calvinism, or something. Instead I am tying this version of Calvinism ('classic') into the system of theology given its first real legs by Thomas Aquinas' synthesis of Christian theology with Aristotelian categories; his integration has become known as classical theism, which spawned the so called analogia entis or analogy of being approach to doing theology. When I use Evangelical in front of Calvinism, I also do this intentionally; and it is derivative of what I consider to be its 'Good News' message, i.e. that God is love and triune, and that he is Sovereign and Free and comes to us in Christ and we have the capacity, through Christ, to participate in his life and answer for life in the life of the eternal Word---this is the kind of positing that supplies us with the 'analogy of faith' approach.
So if you are going to presume that I am naïvely, or unfoundedly pitting classic Calvinism against Evangelical Calvinism; think again. By the way, I realize it's not en vogue for theological types like myself to be so real and impassioned about something; but I actually believe what I am writing, if I didn't, I'd quit writing! Does this kind of topic need more development? Probably, but McCormack has already gone along ways (not to mention Torrance and Barth) in providing a substantial situation from which to draw lines; and so I stand on their shoulders (with a post like this), and refer you to them if you desire a fuller accounting of everything---beyond what my meager out-letting at the blog allows for.