Beheld that which he did (θεασαμενο ο εποιησεν). First aorist middle participle of θεαομα and first aorist active indicative of ποιεω in the relative (ο) clause. They were eye-witnesses of all the details and did not depend on hearsay.

Believed on him (επιστευσαν εις αυτον). Such a result had happened before (John 7:31), and all the more in the presence of this tremendous miracle which held many to Jesus (John 12:11; John 12:17).