I don't believe that the founders of dispensationalism would have agreed that Christians, since the Cross and the Day of Pentecost, are not under the New Covenant.
But saying there are no New Testament texts which point to the New Covenant being established in the First Century is consistent with the very peculiar dispensationalist "Hermeneutic." And here is a verse relevant to this dispensationalist Hermeneutic: "Who also hath made us able ministers of the new testament; not of the letter, but of the spirit: for the letter killeth, but the spirit giveth life." II Corinthians 3: 6
Dispensationalists get the idea that they should support Old Covenant Israel and the Old Covenant, and so if a verse in the New Testament is just a little subtle or nuanced, they decide that such a subtle text is not of the letter, and so does not say that the New Covenant was established in the First Century, which is a certain implication of a number of texts.
Here is a text that tells us about the two covenants and assumes we know that the New Covenant was established as the second covenant: "For it is written, that Abraham had two sons, the one by a bondmaid, the other by a freewoman.
23. But he who was of the bondwoman was born after the flesh; but he of the freewoman was by promise.
24. Which things are an allegory: for these are the two covenants; the one from the mount Sinai, which gendereth to bondage, which is Agar.
25. For this Agar is mount Sinai in Arabia, and answereth to Jerusalem which now is, and is in bondage with her children.
26. But Jerusalem which is above is free, which is the mother of us all." Galatians 4: 22-26
Paul is subtle in talking about the second covenant here in Galatians 4: 22-26 He calls it Jerusalem which is above is free, which is the mother of us all. He is talking about the two covenants in Galatians 4: 22-26, and he gets into explaining the spiritual differences between the two covenants in Galatians 4: 26, saying that the New Jerusalem is free of bondage, is above, or spiritual, and is the mother of us all, that is, New Jerusalem is the mother of us all who are God's elect. Galatians 4: 26 also points to the New Jerusalem in Revelation 21: 2, the bride adorned for her husband, a metaphor.
Here is what Peter warns about Paul's writing: "And account that the longsuffering of our Lord is salvation; even as our beloved brother Paul also according to the wisdom given unto him hath written unto you;
16.As also in all his epistles, speaking in them of these things; in which are some things hard to be understood, which they that are unlearned and unstable wrest, as they do also the other scriptures, unto their own destruction." II Peter 3: 15-16 Paul does not always spell things out in explicit detail. This earning also applies to Romans 11: 25-26
