There is great mystery here. Paul was the human writer of much of the New Testament, yet he also claimed divine inspiration. "I certify you, brethren, that the gospel which was preached of me is not after man. For I neither received it of man, neither was I taught it, but by the revelation of Jesus Christ" (Galatians 1:11-12).
It was only a short time before, however, that Paul had been bitterly opposing that gospel. "Beyond measure," he said, "I persecuted the church of God, and wasted it" (Galatians 1:13). Eventually, he was converted and began to preach "the faith which once he destroyed" (Galatians 1:23). Yet, during all his years of fighting God's truth, he had already been separated unto God and called by His grace even before he was born, as our text reveals. His teachers in the synagogue, his studies under Gamaliel, and even his anti-Christian crusades were all being orchestrated by God to develop Paul into the unique person he would be, the great Christian whom God could use to write much of His own written Word. Paul's epistles were thus truly his epistles, derived from his own experience, research, study, reasoning, and concerns. At the same time, they came out as God's Word, inspired by the Holy Spirit, free from error and perfectly conveyed from God to man, because God had Himself ordained and planned all Paul's experiences and abilities and had implanted all these concerns in his heart.
And so it was with all the human writers of the Bible. God's Word (like Christ Himself) is both human and divine, yet meeting all our needs. This is mysterious indeed, but well within the capabilities of our omnipotent and gracious Creator. HMM