They were not able to withstand (ouk iscuon antisthnai). Imperfect active of iscuw, to have strength, and ingressive second aorist active (intransitive) infinitive of anqisthmi. They continued unable (without strength enough) to take a stand against. Stephen knocked them down, Saul included, as fast as they got up. Stephen was like a battery charged and in action. The wisdom and spirit (th sopiai kai pneumati). Dative case. They stood up against Stephen's wisdom and the Holy Spirit "by whom he spoke" (wi elalei). Instrumental case and the relative agrees with "Spirit." He kept on speaking so (elalei, imperfect active). It was a desperate situation.