A favorite proposition of those who discount New Testament authority goes something like this:
“The apostles had no idea that their teachings would be considered God’s word on par with the Old Testament. The Church imposed that idea onto their writings long after the fact”.
But, as is typical, those who discount scripture tend not to know scripture well enough to make exegetically substantive arguments and they count on your ignorance matching their own.
There are a number of texts indicating that the apostles understood their words to be equivalent to the inscripturated words of Old Covenant prophets. In 1 Thessalonians 2:13 Paul commends the Thessalonian church for receiving his words, not as the words of men but as the words of God. In 2 Peter 3:15-16, Peter refers to Paul’s letters as “scripture”. And in 1st Timothy 5:18 Paul quotes directly from the Gospel of Luke (10:7) referring to the text as scripture. But because of it’s clarity, 2nd Peter 3:2 is perhaps the best place to begin your response:
“you should remember the predictions of the holy prophets and the commandment of the Lord and Savior through your apostles…”(2 Peter 3:2).
Note first that Peter refers to himself as an “apostle of Jesus Christ” in the first verse of this letter (2 Pet 1:1). So when Peter refers to “your apostles” in 2 Peter 3:2, he includes himself in that group.
Second, observe the parity Peter establishes between the “predictions of the prophets” which, as every first century Jew would affirm, come from God and the “commandment of the Lord and Savior through your apostles”. The predictions and the commandments both originate with the Lord.
Third, and this is crucial, just as God spoke through the predictions of the prophets under the Old Covenant – their words being God’s words (see 2 Peter 1:21) – so the Lord and Savior makes his command through “your apostles” among whom Peter has placed himself.
The conclusion is obvious. Peter understood apostolic teaching (his teaching) to be no more “his own” teaching than the predictions of the prophets were their own. The prophets spoke words from God as they were “carried along by the Holy Spirit”(2 Pet 1:21). And so too, in the same way, the Apostles speak words from Christ.
This, of course, does not establish that the teachings of the apostles are the teachings of Christ (that’ would be another argument and another article) but it does establish that Peter (and almost certainly his apostolic colleagues) believed them to be so. For more on this topic, I heartily recommend “The Question of Canon” by Michael J Kruger.