The thought that Christ came to start a new religion, which we now call Christianity, evidently lies at the heart of Christianity itself as a distinct religion. Christians claim to follow Christ, to believe in Him and worship Him, find their salvation in Him, and believe their unique practices and beliefs are what Christ Himself has commanded of them. For these divine instructions they rely exclusively upon the New Testament.
There is, however, a very basic problem with this understanding: Christ Himself never taught this, that He came to start a new religion, neither did any of His apostles. Both Christ and all of His apostles, including the Apostle Paul, believed the one true faith (Ep 4:4) was an old one, largely lost in Judaism yet embodied within the Tanakh, what we now call the Old Testament, which they referred to as the scriptures. (Jn 5:29) There was no concept of a New Testament (NT) scripture during the lifetime of Paul or the Twelve Apostles; they never based any of their teachings on anything but the Tanakh. They had no other scriptural authority, and scriptural authority was all they truly had. (Ac 17:11)
Both Christ (Jn 3:10) and the Apostles taught that the true and correct religion (Ju 3), the way to be in right relationship with God, was the historic faith embodied in the Tanakh. (Ro 16:25-26) Christ’s work and message didn’t alter this in any way, shape or form. (Ps 19:7-9) This is, in fact, how Christ Himself frames His entire ministry: He affirms that His message and redemptive work are grounded in, explained by and perfectly consistent with the Tanakh. (Mt 5:17-19)
While it is evident (at least to me) that the NT writings are just as inspired as the Tanakh, it is also evident that if we’re not properly grounded in the Tanakh we can easily misread, misinterpret, and misapply the NT, particularly the writings of Paul. This is, I believe, the fundamental problem with Christianity as a whole, and it is not a recent problem; it traces as far back as the early second century CE and includes nearly everything which makes Christianity a distinct religion, such as Sunday worship, the Eucharist (the Lord’s Supper), a belief that Christ has abolished or annulled Torah, and a variety of corruptions of the gospel which are deeply inconsistent with the Tanakh. Though considered fundamentals of Christian faith today, such beliefs were foreign to the early Church.
In the earliest days of the Faith, during the Apostolic Age, the disciples of Christ were considered a Jewish sect, The Way (Ac 9:2), a subset of Judaism; gentile believers were largely indistinguishable from their Jewish brothers and sisters in their worship and practice. (Ga 2:14) This sect was distinct from traditional Judaism in two fundamental ways:  a return to justification by faith (rejecting Judaism’s legalism) and  recognizing salvation was available to gentiles even if they didn’t become Jewish proselytes and observe Jewish customs and man-made traditions (as required in Judaism). Apart from these two key differences, the early Church was essentially identical to Judaism; the Church simply corrected the errors of Judaism in these two key areas where she had departed from the way of truth defined in the Tanakh.
It wasn’t until after the death of the apostles, as persecution of the Jewish people become more intense, that a move began to distance the churches from Judaism and from the Tanakh, to redefine the Faith as distinctly non-Jewish. Deceitful workers found plenty of fuel in the Pauline epistles (2Co 11:13), inventing another Jesus, another gospel, and fabricating an entirely new religion. (1Co 11:4)
Peter himself warns us about this, that some of what Paul writes is hard to understand and easy to misinterpret, such that those who are unlearned and unstable typically wrest Pauline statements, as they do also the Tanakh, unto their own destruction. (2Pe 3:16) And Paul himself warned that soon after his departing grievous wolves would enter into the Church, not sparing the flock. (Ac 20:29)
To the degree any Christian sect strays from the Tanakh it will be in error, and when the lies are couched in the very language of scripture, those who are deceived in them are exceptionally difficult to reach, since the words and many of the key concepts are already accepted and believed, but incorrectly, out of context.
This is particularly true of the gospel itself; very few (if any) Christian presentations of the gospel are based on the Tanakh, and what most Christians actually believe about salvation cannot be found within it. In fact, most Christians believe Christ actually came to change the way we’re saved, such that we’re now saved in a different way than those in the old dispensation. Nothing could be farther from the truth, or more eternally dangerous.
As it was for me personally, Christians may indeed find themselves inoculated against the true faith of God, thinking they’re eternally safe when they aren’t, hoping they have spiritual life when they’re still dead in sin. (Re 3:1) It is sobering to realize that many, perhaps most of those complacent in their Christianity will fail to make their calling and election sure (2Pe 1:10), and will be lost in the end, as Christ Himself predicts. (Mt 7:21-23)
Given this tendency to misinterpret and misapply the NT, and the eternal danger this poses, a good litmus test for any Christian teaching about the nature of God or Man (Ro 3:10), or about our duty to God or Man, or about how to be rightly aligned with God and in fellowship with Him (Ro 4:3), is that it must be grounded solidly in the Tanakh. (2Ti 3:16-17) This is following the example of Christ and His apostles; it is exactly what they did. (14-15)
So, if our understanding of the gospel, the ground of our salvation, is not firmly established within the Tanakh (Ro 4:16), and perfectly consistent with it (Ps 119:115), we need to keep seeking and praying until we find God in truth. (Lk 16:27-31) Inundated by counterfeit gospels, the Tanakh makes us wise unto salvation, teaching us what faith in Christ looks like and how to obtain it (1Ti 3:14-15); we cannot afford to be amiss here. (Mt 26:16)
And if any other teaching or doctrine cannot be derived primarily from the Tanakh (Ro 15:4), being only reinforced and supported in the NT, we should hold it loosely, with a bit of suspicion and caution, at arm’s length as it were, and not close to the heart. And, certainly, if any doctrine contradicts or dismisses any part of Torah in any way, we may safely discard it as darkness. (Is 8:20)