First, Jesus used phrases that encompass the whole Old Testament and referred to the Old Testament as a whole (example: Matt. 5:17) but also "beginning with Moses and all the Prophets, he explained to them what was said in all the Scriptures concerning himself' (Luke 24:27). Once He referred to all the books from Genesis to 2 Chronicles (the last book in the Jewish listing of the Old Testament): "And so upon you will come all the righteous blood that has been shed on earth, from the blood of righteous Abel to the blood of Zechariah son of Berekiah" (Matt. 23:35). This is the equivalent of our phrase, "from Genesis to Malachi."
Secondly, Jesus promised the divine authority of the New Testament. Not only did Jesus confirm the Old Testament to be the Word of God, He also promised the same for the New Testament, affirming that the Holy Spirit would teach the apostles "all things" and lead them into "all truth." He announced, "The Counselor, the Holy Spirit, whom the Father will send in my name, will teach you all things and will remind you of everything I have said to you" (John 14:26). He added, "But when he, the Spirit of truth, comes, he will guide you into all truth" (John l6:13).
Note that He said this to the apostles. Jesus' handpicked apostles understood His claims and their role in fulfilling them, for they too claimed to speak with the authority of God, as is evident from the claims made in their books (Gal 1:11-12; 1 Cor. 14:37; 1 John 1:1; 1 John 4:6; 2 Peter 1:16, 18).
He told His apostles to "go and make disciples of all nations, baptizing them in the name of the Father and of the Son and of the Holy Spirit, and teaching them to obey everything I have commanded you. And surely I am with you always, to the very end of the age" (Matt. 28:18-20).
So when someone comes along and says God is leading them into all truth in ways that GO AGAINST THE TEACHINGS OF JESUS AND THE APOSTLES THEY ARE DECEIVED AND/OR LYING. If that happens to be an "entire group of people" rather than just an individual, then the entire group of people are deceived and/or lying.
Today we call these groups cults exactly because they teach heresy that is in opposition to a historical authentic scriptural revelation (e.g. the historical teachings of Christ and the Apostles) and that is exactly what the early church fathers believed and taught. They believed, as their writings reflect, that both the Old and New Testaments were inspired writings of the Holy Spirit through the instrumentality of the prophets and apostles. They also believed the Scriptures were completely true and without error because they were the very Word of God given for the faith and practice of all believers.
Paul wrote of an elder (bishop) that "he must hold firmly to the trustworthy message as it has been taught, so that he can encourage others by sound doctrine and refute those who oppose it" (1:9). He added, "But as for you, speak the things which are proper for sound doctrine" (2:l). The church was to continue faithfully in apostolic teaching (Acts 2:42), and the apostle's delegate was to establish "elders" who would help to preserve this path. There was not to be any contradiction between the apostles and the autonomous, independent local churches they were establishing.
The New Testament is the only authentic historically reliable record of Apostolic teaching. There is some apocryphal literature and a few other manuscripts which make the claim but are excluded for good reason (e.g. forgeries, etc...). The reasons for believing that the twenty-seven books of the current New Testament, and those alone, belong in the Christian New Testament canon are very strong. The evidence includes the promise of Jesus, the providence of God, the preservation by the people of God, and the proclamation of the church.
It may not "feel" good to call to denounce the false teachings of modern day cult leaders, cult founders, false prophets and false prophetesses but that is exactly what the apostles did and what they instruct us in scripture to do and for very good reasons. Consider John MacArthur's four article series on Avoiding Spiritual Counterfeiters:
How to Treat False Teachers, Part 1