The English word "testament" comes from a Latin word. The equivalent Greek word is hard to translate. As used in the Greek Bible it may mean either "covenant" or "testament." Usually it should probably be translated "covenant."
The phrase "new covenant" occurs about five times in the New Testament. In none of these passages does the phrase refer to the "New Testament" in our sense. It designates a new relationship into which men have been received with God. The old covenant was made, through the mediatorship of Moses, with the Hebrew nation; the new covenant, hinted at in prophecy, Jer. 31:31, and instituted by the Lord Jesus, I Cor. 11:25, was made with all those, of every tribe and tongue and people and nation, who should through faith accept the salvation offered by Christ. Those who believe become, like Israel of old, God's chosen people, and enter into the warmth and joy of the divine communion. The names "old and new covenants," then, were applied first to these two special relationships into which God entered with men. Afterwards the names were applied to the books in which the conditions of those relationships were set forth. Perhaps it would have been better if we had started to say "New Covenant" where we now say "New Testament." At any rate the idea alluded to in the name is the inspiring idea, realized in Christ, of an alliance with God. The New Testament is the divine treaty by the terms of which God has received us rebels and enemies into peace with himself.
In the first place, the New Testament may be treated in every respect as a single book. That course is adopted by many of the most devoted lovers of the Bible. By them the Bible is treated simply as a textbook of religion. Passages are quoted indiscriminately from all parts of it, without much regard to the context. The wide differences of form and of spirit among the various books are ignored. The historical implications of the books are of course accepted as true, but practically they are left quite unassimilated.
Now let us be quite plain about one thing. The men who use the Bible in this way are right in the main point. They treat the Bible as the guide of life for time and for eternity. And if by the use of the Bible we can come into communion with God, we can afford to miss a good many other things. Nevertheless, the Bible is as a matter of fact not a mere textbook of religion, and if we treat it as such we miss much of its richness. If the Bible were merely a systematic treatise, it would be far easier to interpret. The interpreter would be spared a great deal of trouble, but the burden would be heaped upon the preacher. As it is, the Bible is itself a preacher, because it is in such close contact with the actual experience of men of flesh and blood. Its general teachings are given us in large measure only through the medium of history, through the medium of example. In order to arrive at the general truths, therefore, intellectual labor is often necessary. God has made things harder for the intellect that he may strike home the more surely to the heart. If Paul had written a systematic theology, the New Testament way of salvation might in some ways have been plainer than it is. It would have been plain to the intellect, but it would have needed interpretation to the heart. Conviction can be wrought only by the immediate impact of personal life. The theology of Paul, of itself, might be a dead thing; the religious experience of Paul, interwoven with his theology, and bared before us in the epistles, is irresistible.