St. Paul knew his scripture well (Scripture at the time would have been just the Old Testament, since they were still busy living the New Testament), and he was very careful in handling verses like today's first reading (Dt 18: 15-20).
Moses' promise that God will raise up "a prophet like me... from among your own kin" is interpreted to mean Jesus, first and foremost. Christ is the Great Prophet, the one who teaches with authority (Mk 1:21-28). The authority of every other teacher comes from the authority of Christ, and Moses' promise applies, too, to all true prophets to follow, which includes Paul.
Paul must have known that people would recognize that he teaches with authority, that the faithful will recognize and listen to a true teacher of God's Word. When Paul speaks his own opinion, then - as he does in today's second reading (1 Cor 7: 32-35) - he is careful to emphasize it. "I should like you...," he writes, and "I am telling you this..."
Why would God allow personal opinion to make it into the Bible? Perhaps it is to teach us about the difference, to give us an example. Paul's opinion is just that - opinion - and we must be careful even today, two thousand years later, to seperate the opinion of clergy from God's revelation.