Do you believe the Gospels?
Much of the Holy Tradition came from the apostles. So if one believes the Gospels and St. Paul, then one automatically believes in Holy Tradition.
In fact the Gospel of John says that not everything is written down. Also one of his letters states a similar line.
So if you believe in the Bible then you have to believe in Holy Tradition too because John and others tell us that not everything was written down.
It seems that you are not well acquainted with the real issues in the debate, as the above is not the issue of contention, but whether being the stewards of revelation confers assured formulaic infallibility. Holding to the supremacy of Scripture does not deny that it was and is a form of "tradition," but that the wholly inspired readings became established as such due to their unique and enduring qualities and the Divine attestation afforded them, just as a true man of God was/is. Because God made Himself real to Abraham and he believed, God supernaturally attested to his faith and overall morality, which established him as a friend of God and through whom a holy nation was born.
Moses was established as "the man of God" due to his holiness and faith which conformed to that which was prior testified to, and to whose authority God mightily supernaturally attested to, and who provided the written law (though Rome's scholars typically hold to the liberal JEDP theory, contrary to what is written). This became the standard by which further revelation was examined by, and likewise as these became progressively established as Holy Writ than they became the standard by which other writings were examined and substantiated by, a principle which is never abrogated. (Is. 8:20; Mt. 22:29-45; Lk. 24:27,44; Jn. 5:39,42; Acts 17:2,11; 18:28; 28:23; Heb. 1, etc.)
By the time of Jesus a distinct body of writing referred to as Scripture was already realized, as is evident by references to the Scriptures, though this was not the result of an infallible magisterium. Later, most of the books of the New Testament were widely accepted as Scripture in a relative short time after they were written, and in the succeeding centuries the only remaining disputed books of were almost exclusively those which were rejected by the Jews, which Trent affirmed as Divine when it provided the “first infallible and effectually promulgated declaration on the Canon of the Holy Scriptures,” (The Canons and Decrees of the Council of Trent (Rockford: Tan, 1978), Fourth Session, Footnote #4, p. 17; http://www.newadvent.org/cathen/03267a.htm; cf. New Catholic Encyclopedia, Vol. II, Bible, III (Canon), p. 390) though this was not exactly the same canon affirmed by such councils as Carthage, but which ended the debate which went on among Roman Catholic scholars right into Trent. (Hubert Jedin, Papal Legate At The Council Of Trent (St Louis: B. Herder Book Co., 1947), pp. 278, 281-282.“ More .
Such decrees can be surely helpful, but they neither make these writings Scripture nor are the real cause for their enduring volitional acceptance, which is due to their supernatural qualities, and the affirmation God gives to those who trust and obey them. Men may decree what they may, but 1 Co. 4:20 is applicable in principle here, “For the kingdom of God is not in word, but in power." and which establishes both men and writings, although i certainly comes short in what i should/could be.
“In fact the Gospel of John says that not everything is written down. Also one of his letters states a similar line.”
True, and to which i more can be referenced, (2Cor. 12:4 Rev. 10:4, etc.), and SS recognizes that, and that Scripture does not contain all there is to know, as it is not “Solo Scriptura,” but as Scripture is the only objective authority which is affirmed to be wholly inspired of God (2Tim. 3:16) thus it alone as the supremer doctrinal authority. And that it is formally sufficient to provide the Truth needed for salvation and holiness, though the body is needed for that truth to be fully effectually, and which church the Scripture materially provides for.
So if you believe in the Bible then you have to believe in Holy Tradition too because John and others tell us that not everything was written down.
Which is just what the Mormon's love to hear, who also invoke this, and interpret history and tradition as validating them.
The problem here is that, unlike Scripture which is a written revelation, often first coming or being soon expressed in writing, (Ex. 7:14; 31:34; Is. 30:8; Jer. 30:2; 36:2) 2Cor. 13:10; Gal. 1:20; Phil. 3:1; 1Thes. 4:9; 2Thes. 3:17; 1Tim. 3:14; Pt. 3:1; 1Jn. 1:4; Jude. 1:3; Rv. 1:1) oral tradition is a nebulous and potential endless source, while for a magisterial office to equate it with Scripture effectively adds to a closed canon, and renders the magister an assuredly infallible and supreme authority over both. Surely the nebulous should judged by the material of God, as God manifested Himself to and as man, and also appealed to Scripture as superior to the Jewish magisterium which presumed to teach unScriptural doctrine. (Mk. 7:6-13; Mt. 22)
The problem then goes back to authority. To reiterate, the authority of men Jesus and the apostles — who added new teachings to an open canon and affirmed that which was establish as Scripture — was established by a holiness and doctrine which conformed to that which was prior established as Scripture, and was mightily attested to by supernatural means. Rome's authority is essentially based upon a claim to formulaic infallibility, that she is speaking infallibility when addressing the whole church, in union with the pope, and which renders her very decree to be infallible, and its criteria infallible. She can claim a Scriptural basis in seeking to justify herself to others, while disallowing that assurance can be had by this means, (contra. 2Tim. 3:15; 1Jn. 3:19; 5:13) but an infallible decree renders her immune to correction, and once one is persuaded to assent to Rome, then implicit trust in her is required. And are admonished against objectively examining what Rome has so defined in order to ascertain its truthfulness, and are also warned about listening to those without (and one time were forbidden to debate.)
“having discovered the authority established by God, you must submit to it at once. There is no need of further search for the doctrines contained in the Christian Gospel, for the Church brings them all with her and will teach you them all.” Henry G. Graham, "What Faith Really Means"
"The intolerance of the Church toward error, the natural position of one who is the custodian of truth, her only reasonable attitude makes her forbid her children to read or to listen to heretical controversy, or to endeavor to discover religious truths by examining both sides of the question." “The reason of this stand of his is that, for him, there can be no two sides to a question which for him is settled; for him, there is no seeking after the truth: he possesses it in its fulness, as far as God and religion are concerned. “The reason of this stand of his is that, for him, there can be no two sides to a question which for him is settled; for him, there is no seeking after the truth: he possesses it in its fulness, as far as God and religion are concerned. were actually once forbidden to engage in debate.” (John H. Stapleton, Explanation of Catholic Morals, Chapter xxiii. the consistent believer (1904); Nihil Obstat. Remy Lafort, Censor Librorum. Imprimatur, John M. Farley, Archbishop of New York)
And once you see yourself as assuredly infallible, you tend to go to further excesses if not restrained. Regarding such, Cardinal Avery Dulles stated that “The interpretation given to infallibility in the present document [Mysterium Ecclesiae] is not something that can plausibly be ascribed to churchmen of the early centuries.”
Sorry if this is too long, but it is a critical foundational issue, and i think it should be comprehensible.