You do understand that there was no need for "rightly dividing God's Word of truth" before Paul was given the gospel of the grace of God, the mystery, to proclaim, don't you? There was no need because no mystery had been proclaimed. EVERYTHING was prophecy. Prophecy being given. Prophecy being fulfilled. That changed when Paul was saved and given the dispensation of the grace of God by the risen Christ. THE MYSTERY. THAT is what had to be rightly divided: Prophecy from the Mystery. Because if those two are not clearly understood and the distinctions clearly made, they become some kind of ball of confused mish-mash, where NOTHING is clear anymore. They are BOTH TRUTH (prophecy and the mystery), but they are DIFFERENT. That's why they must be rightly divided, in order to keep kingdom things, meant for a kingdom of believers from those thing pertaining to the mystery, meant for a body of believers. The entire Bible is for our admonition and learning. But not ALL Scripture is about US. Since we, as members of the Body of Christ, are not the only people God has in His plans and purpose, we must NEVER be so selfish as to think "every promise in the Book is MINE". Knowing WHERE we are in God's plan is the key to understanding God's particular Word FOR US.
The quickest way to put up a wall is to exclaim a promise that isn't yours. And then going about to protect and defend that promise even though Scripture that ARE for you refute your very beliefs. Tongues? There goes a wall between those who DO believe they are for today, and those who do NOT. Visions, miracles, faith healers? Walls go up everywhere. Baptism BY the Holy Spirit, IN the Holy Spirit, THROUGH the Holy Spirit? Walls again. While Paul plainly tells us the is "There is ONE body, and ONE Spirit, even as ye are called in ONE hope of your calling. One Lord, one faith, ONE BAPTISM, One GOd and Father of all, who is above all, and through all, and in you all." (Eph. 4:4-6).
The only way to take down those walls that divide is to rightly divide God's Word of Truth (2 Tim. 2:15). Once His Word FOR US is clearly defined, there is no room for the mish-mash.
There will always be confusion without understanding the “mystery” Paul was given. Most try to place Paul’s teaching back into what Peter was teaching “assuming” Peter meant the same thing as Paul. If they knew nothing of Paul’s teaching they would never place “justification by faith apart from works” into Peter’s teaching.