What was the significance of Paul shaving his head? What could have been the "promise"?
Paul's haircut symbolized his vow as a pious Jew to maintain the purity of his consecrated relationship with God. Although it is mentioned only in passing here, it was probably a Nazirite vow described in Numbers 6:1-21. If that is the case, the ritual haircut represents his dedicated service for God and within Israel (Numbers 6:9, 18). Normally such rituals were performed in Jerusalem, but they were sometimes allowed outside of the holy city if the consecrated person then went to Jerusalem on a pilgrimage to the Temple for priestly confirmation. In the Book of Acts, vow taking is an indication that Paul's ministry among uncircumcised Gentiles had contaminated neither his loyalty to the traditions of his Jewish faith nor his vocation as a teacher of Israel.