What did He give "these twelve [whom] Jesus sent forth"? What is an apostle? It is one sent forth with a message. Thinking about the principle in Romans 10:17, that faith comes by hearing the word of Christ, Jesus gave the same words to those He sent forth! They are the ones who have the message that will produce saving faith!
When we read about fracturing of the church during the first century—in the books of James, I and II Peter, I, II, and III John, and Jude—we find direct and indirect references, sometimes very strong, in which the apostle writes, "Remember what we have taught you." Other messages were coming into the church, and people were falling for them because they were susceptible to them—they were too weak to reject them and to discern the deceit in them. They believed them, and then what was the result? Disobedience. This factor separates those who believe from those who do not. Those who believe will obey God. Those who do not believe will not obey Him because "the carnal mind is enmity against God, for it is not subject to the law of God, neither indeed can be" (Romans 8:7).
We find ourselves in a battle, a struggle, between the carnality that remains, which is attracted by false messages, and the truth of God, which is the right message, the proper faith. Paul describes it in Galatians 5:17 as a war going on in us (see also I Peter 2:11). By the power of God's spirit, we have to make the choice as to which one we will submit to.