As theology grew and expanded in depth and scope, theologians often found that they had to make finer and finer distinction between the various shades and nuances of the terms they use. These two Latin phrases arose out of the need to distinguish such distinctions with regards to the use of the term "faith." Faith can, first, be understood as the sum of our relationship with God, of our trust in Him and His promises, and what we know at a deep personal level what He could and would do, and what pleases Him, and, therefore, how we order our lives accordingly. Theologians used the phrase Fides qua creditur to refer to this, "the faith which one believes." But faith can also refer to the sum of "what" we believe about Him as summarised and affirmed in our doctrines, creeds and traditions (as well as, as often happens, a bag of superstitions). This is "the faith by which one believes," or what theologians call fides quae creditur. In real life, of course, the two are inseparable: "Fides qua without fides quae is emotionalism with all sorts of heartfelt sentiments but no understanding of precisely what Jesus is all about. Fides quae without fides qua is heartless theological abstractions—what may father used to call 'theological ivory-towerism' annd logomachists" (Tim Rossow, steadfastluterans.org).
Now a friendly reminder: as preachers, you may need to know this stuff, but don't go showing off by dumping this on your congregation. If you ever find the need to make such a distinction for your flock at all, it can be done in simpler ways.
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