When Paul told the elders that they would see his face "no more," or perhaps rather "no longer," Acts 20:25,38, he did not necessarily mean that he would certainly never return to Ephesus. For a period of years, at any rate, he was intending to transfer his labors to the west; his return to Ephesus, therefore, was at all events uncertain. His long activity at Ephesus, which had occupied the better part of the past three years, was for the present at an end. From the Pastoral Epistles it appears that as a matter of fact Paul did visit Ephesus again after his release from the first Roman imprisonment.
At Tyre and at Cæsarea, Paul received warnings against visiting Jerusalem. These warnings came through the Spirit, Acts 21:4,11, but not in the sense that the Holy Spirit commanded Paul not to go. The meaning is that the Spirit warned him of the dangers that were to befall him. In meeting these dangers bravely he was acting in full accordance with the divine will.
At Acts 21:18 the use of the first person plural ceases, because Luke had no immediate part in the events that followed. It is natural to suppose, however, that he remained in Palestine, for he joined Paul again in Cæsarea, at the beginning of the journey to Rome. For the events of Paul's imprisonment in Jerusalem and in Cæsarea he had first-hand information.
The vow in which Paul took part at the request of James was at least similar to the Nazirite vow described in Num. 6:1-21. Not all the details of such vows are perfectly clear. Paul himself, on his own account, had assumed a similar vow on his second missionary journey, Acts 18:18--unless indeed, as is grammatically possible, the words in that passage refer to Aquila rather than to Paul.
It was not true, as the Christians of Judea had been led to think, that Paul taught the Jewish Christians of the dispersion to forsake the law of Moses, though he was insistent that the Gentile Christians must not adopt that law. It was not even true that he himself had altogether given up keeping the law, though the exigencies of his Gentile work required him to give it up very often, and though he regarded himself as inwardly free from the law. His willingness to take part in a Jewish vow in Jerusalem is therefore not surprising. His action on this occasion was fully justified by the principles of his conduct as described in I Cor. 9:20,21. The keeping of the law was not for Paul a means of obtaining salvation. Salvation was a free gift of God, through the death of Christ. But for the present the general relinquishment of the law and abandonment of the distinctive customs of Judaism on the part of Jewish Christians was not required. Paul was willing to leave that question to the future guidance of God.