Suppose that you are a liberal critic of the Bible. By liberal, I mean unfettered by commitment to the inerrancy and authority of the Bible. And suppose that you find in an early letter of the apostle Paul these words: "Concerning you, my brethren, I myself also am convinced that you yourselves are full of goodness, filled with all knowledge." And then suppose that you find in a letter written near the end of his life the words, "You are able to admonish one another." And suppose that, as a liberal, you are not inclined to find old-fashioned unity in Paul's various teachings, but rather are somewhat excited when you can construct new theories about how the diversity of Paul's teachings emerged.
So you infer that there was an "early Paul" who was enthusiastic and optimistic and perfectionistic. And you prove it with the early words, "You yourselves are full of goodness, filled with all knowledge." You argue that "full of goodness" and "filled with knowledge" clearly imply that Paul believed these people "had arrived." They had reached a state of perfection. They didn't need any teaching or any correction. They were "full of goodness." This is the "early Paul." But you also infer that Paul's thinking developed over time. He changed his mind and the "later Paul" emerged. Reality had settled in over the years, and Paul's optimism had been dashed by people's imperfections; and so he had adjusted his theology to something more realistic.
Perfectionism had given way to process. And you prove it with the later words, "You are able to admonish one another." You reason, "Clearly, if they had to admonish one another, they were not yet perfect." In fact, with scholarly flourish, you observe that the verbs "are able" and "to admonish" are both in the present continuous tense, implying ongoing action. And you argue that the imperfections must be fairly constant, because they require continuous admonition and correction.
From all this, you proclaim, with liberal "courage," over against conservative commitments to the inspiration and coherence of all apostolic writings, that Paul cannot be inspired by an all-knowing God, and his writings are limited and sometimes mistaken by his ordinary human perspective. He has to correct himself when experience proves his earlier efforts erroneous.
Now here is one of the problems with such an imaginary scenario. Romans 15:14 combines both of those words (the so-called early and the so-called later) in one verse: "And concerning you, my brethren, I myself also am convinced that you yourselves are full of goodness, filled with all knowledge and able also to admonish one another." In other words, there is nothing, in Paul's mind, inconsistent in saying both of these things about the same people at the same time. Being "full of goodness" and "filled with all knowledge" is not meant to imply perfection. It does not mean that the people are beyond the need for admonition and correction. The "fullness" is not a fullness of sinless perfection, but fullness of sufficiency for ministry, that is, the church in Rome had all the goodness and knowledge it needed to minister effectively to each other through admonition and correction.
Here's the point. When you find two parts of Scripture that may seem to be in tension with each other, don't make the liberal mistake of jumping to the conclusion that the Bible is inconsistent or self-correcting or in process of moving from enthusiastic error to realistic truth. Instead, picture the things that seem in tension as spoken by a person who means both of them, and regards both of them as true. Then work toward a coherent understanding of them as best you can. This will take you much deeper into the reality of God's truth. And you will honor the divine Author of Scripture.
Loving God's inerrant word with you,