Albert Barnes Commentary Acts 21:26

"Then Paul took the men, and the next day purifying himself with them went into the temple, declaring the fulfilment of the days of purification, until the offering was offered for every one of them." — Acts 21:26 (ASV)

Then Paul took the men. He took them to himself, uniting with them in observing the ceremonies connected with their vow. To transactions like this he refers in 1 Corinthians 9:20: And unto the Jews I became as a Jew, that I might gain the Jews; to them that are under the law, as under under the law.

Thus, it has always been found necessary in propagating the gospel among the heathen not to offend them needlessly, but to conform to their innocent customs regarding dress, language, modes of travel, sitting, eating, and so on. Paul did nothing more than this; he violated none of the dictates of honesty and truth.

Purifying himself with them means observing the ceremonies connected with the rite of purification. (See the comments on Acts 21:24.)

This evidently means that he entered on the ceremonies of separation according to the law of the Nazarite.

To signify (Greek: signifying or making known) means that he announced to the priests in the temple his purpose of observing this vow with the four men, according to the law respecting the Nazarite. It was proper that such an announcement should be made beforehand, so that the priests would know that all the required ceremonies had been observed.

The accomplishment, etc. (meaning the fulfilling or completion) indicates that he announced to them his purpose to observe all the days and all the rites of purification required in the law, so that an offering might be properly made. It does not mean that the days had been accomplished, but that it was his intention to observe them so that it would be proper to offer the usual sacrifice.

Paul had not, indeed, engaged with them in the beginning of their vow of separation, but he could join in with the sincere intention to share with them. It cannot be objected that he meant to deceive the priests and make them believe that he had observed the whole vow with them. For it appears from their own writings (Bereshith Rabba 90, and Koheleth Rabba 7) that in those instances where Nazarites did not have sufficient property to meet the whole expense of the offerings, other, wealthier persons could become sharers in it, and thus be made parties to the vow.

See Jahn's Archeology, section 396. This circumstance vindicates Paul from any intention to take an improper advantage or to deceive the priests or the Jews. All that he announced was his intention to share with the four men in the offering they were required to make and to divide the expenses with them. In this way, he showed his approval of the practice and his accordance with the law that made such a vow proper, as he had done before in a voluntary manner, when it could not be pretended that it was for double-dealing or deception (Acts 18:18).

Until that an offering, etc. refers to the sacrifices required of all those who had observed this vow. (See the comments on Acts 21:24 and Numbers 6:13.)

It is a complete vindication of Paul in this case that he did no more here than he had done voluntarily (Acts 18:18), and, as it appeared at the time, in a secret manner, showing that he was still practicing this rite of the Mosaic institution. Nor can it be proved that Paul ever, in any way or at any time, spoke against the vow of the Nazarite, or that a vow of a similar kind in spirit would be improper for a Christian in any circumstances.