"Lest my detractors hurl accusations of 'quoting out of context', and/or 'inaccuracy' in my direction, I want to make it quite clear from the start that Luther WAS NOT questioning the 'doctrine of the Trinity'; rather, he was advancing the notion that it is better to stick with Scriptural terminology, and refrain from non-Scriptural language."
If one simply searches Luther's Works for the word "Trinity," you would never arrive a the conclusion "it is better to stick with Scriptural terminology, and refrain from non-Scriptural language" in regard to his use of the term. In almost every instance I've read from Luther using the word "Trinity" it is positive. If Luther did want to avoid the word, it appears to only be in isolated instances, for in fact he used the term throughout his career. I think Luther realized the church was stuck with the term despite its inadequacy to describe the being of God. For instance,
1. This epistle is read today because the festival of Holy Trinity, or of the three persons of the Godhead--which is the prime, great, incomprehensible and chief article of faith--is observed on this day. The object of its observance is that, by the Word of God, this truth of the Godhead may be preserved among Christians, enabling them to know God as he would be known. For although Paul does not treat of that article in this epistle, but touches on it only in a few words in the conclusion, nevertheless he would teach that in our attempts to comprehend God we must not speculate and judge according to human wisdom, but in the light of the Word of God alone. For these divine truths are too far above the reach of reason ever to be comprehended and explored by the understanding of man.
2. And although I have, on other occasions, taught and written on this article fully and frequently enough, still I must say a few words in general concerning it here. True, it is not choice German, nor has it a pleasing sound, when we designate God by the word "Dreifaltigkeit" (nor is the Latin, Trinitas, more elegant); but since we have no better term, we must employ these. For, as I have said, this article is so far above the power of the human mind to grasp, or the tongue to express, that God, as the Father of his children, will pardon us when we stammer and lisp as best we can, if only our faith be pure and right. By this term, however, we would say that we believe the divine majesty to be three distinct persons of one true essence. [Complete Sermons of Martin Luther Volume 4.2, pp. 7-8]
See also: Luther Condemned the Word "Trinity"? (1/28/10)