Do you honestly believe that God would omit anything needful to us out of his inspired book? Do you really have such a low estimation of His ability to successfully record everything for His kids?
Paul was an Apostle, called by God, to reveal the Mystery of the Body of Christ. He did not give them man-made traditions. He taught them exactly what God instructed, and recorded everything Believers need to know. In this case in particular, he is writing to correct religious error that was undermining God's Truth - that the Day of the Lord was at hand.
The chapter starts with a warning - let no one deceive or trouble you. Paul then shares with them the sequence of events leading up to the return of Christ, contrasting those who are damned for not believing the Truth, with the Believers who are sanctified by the Truth. What is Truth? God's Word is Truth.
2 Thessalonians 2:14-15 (KJV)
14 Whereunto he called you by our gospel, to the obtaining of the glory of our Lord Jesus Christ.
15 Therefore, brethren, stand fast, and hold the traditions which ye have been taught, whether by word, or our epistle.
How were the "traditions" taught by Paul - by word (logos) or our epistle. The Greek word paradosis means tradition or instruction. For it to mean what Rome desires, that would mean that there was teaching that Paul, or even worse, the Holy Spirit, left unwritten for the Body of Christ. Do you really believe this the case? I don't because, All scripture is given by inspiration of God... That the man of God may be perfect, throughly furnished unto all good works. (2 Timothy 3:16-17) How can a Believer be perfect or complete, and equipped for every good work if the Holy Spirit neglects something vital to our spiritual growth?
Mark 7:3-9 (KJV) - 3 For the Pharisees, and all the Jews, except they wash their hands oft, eat not, holding the tradition of the elders. 4 And when they come from the market, except they wash, they eat not. And many other things there be, which they have received to hold, as the washing of cups, and pots, brasen vessels, and of tables. 5 Then the Pharisees and scribes asked him, Why walk not thy disciples according to the tradition of the elders, but eat bread with unwashen hands? 6 He answered and said unto them, Well hath Esaias prophesied of you hypocrites, as it is written, This people honoureth me with their lips, but their heart is far from me. 7 Howbeit in vain do they worship me, teaching for doctrines the commandments of men. 8 For laying aside the commandment of God, ye hold the tradition of men, as the washing of pots and cups: and many other such like things ye do. 9 And he said unto them, Full well ye reject the commandment of God, that ye may keep your own tradition.
See also - Matthew 12:1-8, 15:1-20, 23:1-23, Luke 6:1-11
There is a contrast in traditions that come from God, and from men. And it appears this is most common within religious circles.
Colossians 2:6-12 (KJV) - 6 As ye have therefore received Christ Jesus the Lord, so walk ye in him: 7 Rooted and built up in him, and stablished in the faith, as ye have been taught, abounding therein with thanksgiving. 8 Beware lest any man spoil you through philosophy and vain deceit, after the tradition of men, after the rudiments of the world, and not after Christ. 9 For in him dwelleth all the fulness of the Godhead bodily. 10 And ye are complete in him, which is the head of all principality and power: 11 In whom also ye are circumcised with the circumcision made without hands, in putting off the body of the sins of the flesh by the circumcision of Christ: 12 Buried with him in baptism, wherein also ye are risen with him through the faith of the operation of God, who hath raised him from the dead.
Spiritually reborn, you are complete in Christ. You have the Holy Spirit dwelling in you to guide and comfort you, and God's Truth to teach you and increase your faith. What could any man-made tradition do for you?
1 Peter 1:18-25 (AMP) - 18 You must know (recognize) that you were redeemed (ransomed) from the useless (fruitless) way of living inherited by tradition from [your] forefathers, not with corruptible things [such as] silver and gold, 19 But [you were purchased] with the precious blood of Christ (the Messiah), like that of a [sacrificial] lamb without blemish or spot. 20 It is true that He was chosen and foreordained (destined and foreknown for it) before the foundation of the world, but He was brought out to public view (made manifest) in these last days (at the end of the times) for the sake of you. 21 Through Him you believe in (adhere to, rely on) God, Who raised Him up from the dead and gave Him honor and glory, so that your faith and hope are [centered and rest] in God. 22 Since by your obedience to the Truth through the [Holy ] Spirit you have purified your hearts for the sincere affection of the brethren, [see that you] love one another fervently from a pure heart. 23 You have been regenerated (born again), not from a mortal origin (seed, sperm), but from one that is immortal by the ever living and lasting Word of God. 24 For all flesh (mankind) is like grass, and all its glory (honor) like [the] flower of grass. The grass withers and the flower drops off, 25 But the Word of the Lord (divine instruction, the Gospel) endures forever. And this Word is the good news which was preached to you.
If the Word of God is incorruptible seed, what is the word of man? What are his traditions, ceremonies, and rituals?
Matthew 15:6-9 (AMP) - 6 So for the sake of your tradition (the rules handed down by your forefathers), you have set aside the Word of God [depriving it of force and authority and making it of no effect]. 7 You pretenders (hypocrites)! Admirably and truly did Isaiah prophesy of you when he said: 8 These people draw near Me with their mouths and honor Me with their lips, but their hearts hold off and are far away from Me. 9 Uselessly do they worship Me, for they teach as doctrines the commands of men.
What did Paul teach the Thessalonian church? The revelation of Jesus coming for His Body might be too much for spiritual babes. However, the Believers at Thessalonica were a model church. They had given up idols, turned to the Living God, and were waiting for the return of Jesus. They believed the Word and became imitators of the Lord, spreading their faith throughout Achaia (Greece) and Macedonia. (1 Thessalonians 1:3-10) They received Paul’s teaching as the Word of God, and put it into practice even in the face of persecution. (1 Thessalonians 2:7-16) They stayed true to the Word, growing in love, even after Paul’s departure. (1 Thessalonians 4:9-12) They were primed and uniquely prepared to receive even more revelation on God’s plan for His Church. Paul's second letter was correcting religious error that was undermining their faith in what he had already taught. They were not instructed in any secret handshakes or mystic rituals. They received the Truth from God through Paul, and the Holy Spirit confirmed it with their spirit. Nothing was omitted that is needful to the Believer.
In one of Paul's last letters he spells out what is important, and the danger when people neglect the Word and turn to religion. God's Truth sets people free. Religion puts them in bondage.
2 Timothy 4:1-4 (AMP)
1 I CHARGE [you] in the presence of God and of Christ Jesus, Who is to judge the living and the dead, and by (in the light of) His coming and His kingdom:
2 Herald and preach the Word! Keep your sense of urgency [stand by, be at hand and ready], whether the opportunity seems to be favorable or unfavorable. [Whether it is convenient or inconvenient, whether it is welcome or unwelcome, you as preacher of the Word are to show people in what way their lives are wrong.] And convince them, rebuking and correcting, warning and urging and encouraging them, being unflagging and inexhaustible in patience and teaching.
3 For the time is coming when [people] will not tolerate (endure) sound and wholesome instruction, but, having ears itching [for something pleasing and gratifying], they will gather to themselves one teacher after another to a considerable number, chosen to satisfy their own liking and to foster the errors they hold,
4 And will turn aside from hearing the truth and wander off into myths and man-made fictions.
In order to avoid wandering after man-made doctrines, God gave us His Word, and the Holy Spirit to guide Believers and confirm His truth.
Here,
I'm not making the claim that anything needful was omitted, it wasn't omitted. It's been passed down for 2000 years exactly the same as it was when the Apostles taught it and written down in many forms. In fact, Catholics keep the entire Bible as being "needful". Protestant and Protestant who don't want called Protestant folks, on the other hand, throw a large portion of the Old Testament in the garbage can and in doing so blaspheme the Holy Spirit.
The claim that everything "needful" is in the anti-Christ, anti-Christian, Pharisee Approved, Luther Subset of Scripture is the issue and it is not I who makes that claim. The Self and Self Alone crowd do claim that Solo Scriptura is foundational and there is no need for anything but the subset of Scripture they claim to accept.
As a result of their applying that doctrine, they've changed what they teach as the truth every time the society they're in pressures them to go along to get along or they feel left out as the society they're in changes what it does or doesn't accept.
And the garbage, "do you have such a low estimation" trash may be funny for the Sola Yourself peanut gallery but it's a crystal clear indication that the person using that sort of little ploy knows exactly how nearly, if not totally, indefensible their position is without diversions and word games.
Someone who throws out part of the Bible and then implies that those who don't do the same think something is missing from His Word is either just a stupid trained seal with a ball on their nose, or under a strong delusion that's leading them to their own destruction.
have a nice day