Joseph Smith gives three closely related keys.
1. The twelve are not sent out to be taught, but to teach.
2. That in all our trials and troubles do not betray heaven, that is, we do not betray Jesus Christ, do not betray the brethren, do not betray the revelations that were given at any time.
3. That man who condemns the brethren saying they our out of the way but he is righteous is on the high road to apostasy, and will apostatize if he does not repent.
This last is stated as one of the keys of the mysteries of the kingdom.
Here are Joseph Smith's actual statements, all found on page 156 of Teachings of the Prophet Joseph Smith:
Then, O ye Twelve! notice this Key, and be wise for Christ's sake, and your own soul's sake. Ye are not sent out to be taught, but to teach. Let every word be seasoned with grace. Be vigilant; be sober. It is a day of warning, and not of many words. Act honestly before God and man. Beware of Gentile sophistry; such as bowing and scraping unto men in whom you have no confidence. Be honest, open, and frank in all your intercourse with mankind.
O ye Twelve! and all Saints! profit by this important Key--that in all your trials, troubles, temptations, afflictions, bonds, imprisonments and death, see to it, that you do not betray heaven; that you do not betray Jesus Christ; that you do not betray the brethren; that you do not betray the revelations of God, whether in Bible, Book of Mormon, or Doctrine and Covenants, or any other that ever was or ever will be given and revealed unto man in this world or that which is to come. Yea, in all your kicking and flounderings, see to it that you do not this thing, lest innocent blood be found upon your skirts, and you go down to hell. All other sins are not to be compared to sinning against the Holy Ghost, and proving a traitor to the brethren.
I will give you one of the Keys of the mysteries of the Kingdom. It is an eternal principle, that has existed with God from all eternity: That man who rises up to condemn other, finding fault with the Church, saying that they are out of the way, while he himself is righteous, then know assuredly, that that man is in the high road to apostasy; and if he does not repent, will apostatize, as God lives. The principle is as correct as the one that Jesus put forth in saying that he who seeketh a sign is an adulterous person; and that principle is eternal, undeviating, and firm as the pillars of heaven; for whenever you see a man seeking after a sign, you may set it down that he is an adulterous man.