My answer to the OP question is that no born again person can "lose" their salvation, but any can abandon/reject Christ, making shipwreck of their saving faith, with no hope of eternal life. Paul reached out to his Jewish peers who had no further excuse after reading the epistle of "Hebrews". They couldn't discount the vital blood of Jesus as the final accepted sacrifice, putting above that flesh circumcision and law-keeping, and remain Christian. It's a matter of history that most of them fell back into unbelief as well as failed to be successful with their old dogmas.
The Church which remained within the walls of Jerusalem, worshiping in the Temple areas allowed to them, tended to add the Gospel of Christ to the Mosaic Covenant such that the only differences between them and Jews as naming Christ as Messiah, and adding Sunday to the last day of the week Jews worshiped on. They mixed Moses and Jesus, and it is solidly understood that the Church at Jerusalem became powerless through that mixing of covenants unto the destruction of Jerusalem and the Temple in 70 AD. That ended the Church at Jerusalem, as citizens of Jerusalem and Israel at large were dispersed by Rome. That didn't stop such "Judaizers" (the Circumcision, a sect of Christianity) from continuing to insist on a mixing of the two covenants, even after they agreed to stop in Acts 15. After all, were not Christians heirs of Abraham's promise? Those earliest Jesus believers who wanted the Law of Moses to be added were Paul's "thorn in the flesh" all his life, throughout his missionary journeys. Paul remained steadfast, leading the other apostles to understand why the Mosaic Covenant had been abolished in the blood of Jesus, which ratified the New Covenant. Begin with thinking out Ephes. 2:15 "Having abolished in his flesh the enmity, even the law of commandments contained in ordinances; for to make in himself of twain one new man, so making peace; "
to understand that the Mosaic Covenant stood a long time to preserve the Jews as a peculiar people until Jesus the Christ should come to make peace with men and God.
The word "sect" simply means there were and remain different groups holding different dogmas based on scriptures and traditions, giving rise to the many denominations of Christianity today. It isn't an inherently bad term, used to identify disagreements between religious communities. Acts 15 proves there were at least two different sects of Christianity, requiring a compromise between them. Ever since then judaizers have attempted to increase the Moses portion.
The souls that Paul, John, and eventually Peter and the other apostles sought to win were among pagans, mostly citizens of Asia Minor, chiefly in what we today call modern Turkey. Judaism was a very strange religion to them. However, they far more readily accepted the concept of Jesus being in the class of deity, while the Judo-Christians tended to downgrade Jesus to status of prophet, much like Muhammad persuaded his followers to think. Muslims accept Jesus as a prophet among prophets, a holy man, but not deity. Paul's teachings defied that, resulting in a "Gentilized Church" that eventually and very substantially replaced the Judao-Christian philosophy, promoting the truth that Jesus was indeed One of the Triune Godhead. However, with Paul having his roots as a Pharisee, a master of the Law of Moses, and a Jew, he didn't abandon the Jew-Church, writing at least the one epistle, that marvelous letter to them, called Hebrews. There remains no further argument for convincing those precious Jewish souls to completely convert over to Jesus, while appreciating Moses for bringing them to Christ.
The LAW is holy and just, and yes, Jesus never condemned it.....but...... He replaced it with the Law of Life and Liberty, the NEW COVENANT. That new COVENANT is better than the old commandments, the new wineskin better than a dried out cracked old wineskin that leaked.
Jim