I think you misunderstand, the new covenant is VERY explicit :-
.. they shall NOT teach every man his neighbour,
and every man his brother,
saying, Know the Lord:
for all shall know me,
from the least to the greatest.
How does every saint come to know God and be sinless in THIS world, simply by baptism of the spirit as Jesus received , without measure...
Thus sound doctrine is GIVEN by God , whereas many doctines of the traditions of men are even contrary to some scripture ...
Thus what Jesus and Paul instructed their disciples to do was just to go to the lost sheep of the House of Israel with the 'good news that the new covenant with Israel is in force because Israel has a new King [annointed , Christ, messiah] to re-unite the two Houses of Israel again into ONE holy nation, the people of God, the descendants [Children] of Israel re-united as one nation to be a kingdom of priests under the new covenant, priests of the order of Melchizedek, not Levites, the whole nation....
Exodus 19:6 And ye shall be unto me a kingdom of priests, and an holy nation. These are the words which thou shalt speak unto the children of Israel.
This then is what God sent Jesus THIS time to do, it is what Jesus said that he was sent to do... re-establish the identity of the paganised House of Israel to them , that is why he and Paul sent the message out amongst the gentiles, because the House of Israel was there amidst the gentiles, thinking they were gentiles, not long lost descendants of Jacob
Amos 9:9 For, lo, I will command, and I will sift the house of Israel among all nations, like as corn is sifted in a sieve, yet shall not the least grain fall upon the earth
This simple example itself shows how far the traditions of men ,called 'doctrines/dogmas./ have strayed from sound doctrine without the baptism of the spirit in modern churches that gives ALL truth of God and knowledge of prophecy... instead of being taught by God through the holy ghost, men study the traditions of men mostly and then try to make the scripture fit, simply ignoring where it does not fit the ideas of tradition
Colossians 2:8 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
The holy spirit WOULD unify men into ONE belief of God, so the fact that christianity is divided into tens of thousands of sects and beliefs actually PROVES the absence of the holy spirit from most of christianity... that would indeed be a shocking thing were it not prophesied that the church would fall away [2 Thess 2] , but it hardly justifies any faith placed by people in their self-ordained unannointed priests [who can hardly follow Jesus without the holy spirit
John 16:13 Howbeit when he, the Spirit of truth, is come, he will guide you into all truth for he shall not speak of himself; but whatsoever he shall hear, that shall he speak: and he will shew you things to come
Ephesians 5:9 (For the fruit of the Spirit is in all goodness and righteousness and truth
So where is even one man who knows ALL the truth of God after 2 millenia from Jesus the man? ... ? - Surely someone has to notice that the church is not given men all truth because it does not have it to give, but is divided and uses the methiods of MEN to divide its inerpretations of scripture , not the one truth of the holy spirit to unite it, but Jesus promised ALL truth, ONE truth of God to HIS followers..
So, those WITH the spirit of truth and all truth of God CAN teach God's truth, but that just is NOT anyone who has NOT all truth of God... which means almost all people and in particular almost all priests, preachers, evangelists, missionaries
So man is just leading man astray on the whole, the blind leading the blind down lines of men's traditions that could be set straight from scripture, but they are not
2 Timothy 3:16 All scripture is given by inspiration of God, and is profitable for doctrine, for reproof, for correction, for instruction in righteousness:</p> <p
Again scripture is a way to be taught of God indirectly... the main opponent though is traditions in the churches which have gone down lines of personal beliefs
Peter 1:20 Knowing this first, that no prophecy of the scripture is of any private interpretation
If scripture then must all be true [or the scripture should not be in the 'canon'] then it is a great sin to just ignore the parts that do not fit with one's preferred tradition... rather one must INTEGRATE the WHOLE scripture into ONE ... but then one gets a VERY DIFFERENT view of what Jesus was sebnt by God to do
Matthew 15:24 But he answered and said, I am not sent but unto the lost sheep of the house of Israel
and what Jesus and Paul sent their disciples to do , the same thing
Matthew 10:5 ¶ These twelve Jesus sent forth, and commanded them, saying, Go not into the way of the Gentiles, and into any city of the Samaritans enter ye not:6 But go rather to the lost sheep of the house of Israel
Acts 2:36 Therefore let all the house of Israel know assuredly, that God hath made that same Jesus, whom ye have crucified, both Lord and Christ [note - Messiah, annointed King of Israel ]
The gospel too has been lost, yet Paul re-iterates above that the good news is that Jesus is the Messiah [Christ, annointed King] of the new united Israel [when the two Housaes arejoined to be a nation of priests by menas of the new covenant with Israel
and the new covenant itself is hardly recognisable in churches, the original is still the ONLY new covenant between God and men and it is ONLY with the House of Israel [paganised as gentiles] and the House of Judah [the Jews]
Heb 8:6 ¶ But now hath he obtained a more excellent ministry, by how much also he is the mediator of a better covenant, which was established upon better promises
7 For if that first covenant had been faultless, then should no place have been sought for the second
8 For finding fault with them, he saith, Behold, the days come, saith the Lord, when I will make a new covenant with the house of Israel and with the house of Judah
9 Not according to the covenant that I made with their fathers in the day when I took them by the hand to lead them out of the land of Egypt; because they continued not in my covenant, and I regarded them not, saith the Lord
10 For this is the covenant that I will make with the house of Israel after those days, saith the Lord; I will put my laws into their mind, and write them in their hearts: and I will be to them a God, and they shall be to me a people
11 And they shall not teach every man his neighbour, and every man his brother, saying, Know the Lord: for all shall know me, from the least to the greatest
12 For I will be merciful to their unrighteousness, and their sins and their iniquities will I remember no more
13 In that he saith, A new covenant, he hath made the first old. Now that which decayeth and waxeth old is ready to vanish away
Look at what the ACTUAL new covenant SAYS, that even though Israel has sinned, they will be UNCONDITIONALLY both forgiven and TAKEN as God's people... yet in tradition in churches it is often raught that Israel 'blew it' and are no longer going to be god's people, His firstfruits of redemption after Jesus
And Jesus LISTS the firstfruits, tribe by tribe in Rev 7:3-8 and calls them the servants of God , sealed by menas of ALL God's truth, by the holy spirit of truth, the Comforter, just as Jesus was... but people still believe that they will be the first and scripture shows that no-one but those few given by God to jesus CAN travel the narrow way to the first resurrection ... thus the TEACHING has to lead to the FULL baptism of the spirit and ALL knowledge of God... and at the moment that just doen't happen, does it !,,, men are NOT receiving the holy spirit as Jesus did, me cannot do what jesus and the disciples did, the miracles and so forth... these things TELL us that doctrine has gone astray... we all need to be taught of God by his spirit, because that is where the good news should lead anyone who will, can, follow jesus worthily... and to follow unworthily is to crucify Jesus all over again
1 Cor 11:27 Wherefore whosoever shall eat this bread, and drink this cup of the Lord, unworthily, shall be guilty of the body and blood of the Lord
28 But let a man examine himself, and so let him eat of that bread, and drink of that cup
29 For he that eateth and drinketh unworthily, eateth and drinketh damnation to himself, not discerning the Lords body
30 For this cause many are weak and sickly among you, and many sleep