Christ and His apostles defined the New Covenant—don't try to re-define it!

Update: I believe that the New Covenant is defined by Christ's words, and that other words in the Bible can support understanding of Christ's words. However, contradictions and tensions can also be found. Please see my Translator's Preface to The Christ Family Bible.

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Also in ancient times, people depended on agreements. The most dependable agreement was (and still is) the agreement that God offers to humanity. This is because God is not only all-powerful, able to perform what He promises, but also perfectly honourable. God has exalted His word above all His name (Ps. 138:2, cp. Num. 23:19, Deut. 7:9).

What is the New Covenant in Christ? What is the agreement here? That is what the New Testament is for. Its 27 books define the agreement. 

What then can we say and observe of people who claim that the New Covenant in Christ can be defined differently?