acts 17:23 For as I passed by, and beheld your devotions, I found an altar with this inscription, TO THE UNKNOWN GOD. Whom therefore ye ignorantly worship, him declare I unto you. - Free Bible Online

      23. as I passed by and beheld your devotions--rather, "the objects of your devotion," referring, as is plain from the next words, to their works of art consecrated to religion.
      I found an altar . . . To the--or, "an"
      unknown god--erected, probably, to commemorate some divine interposition, which they were unable to ascribe to any known deity. That there were such altars, Greek writers attest; and on this the apostle skilfully fastens at the outset, as the text of his discourse, taking it as evidence of that dimness of religious conception which, in virtue of his better light, he was prepared to dissipate.
      Whom therefore ye ignorantly worship--rather, "Whom, therefore, knowing Him not, ye worship," alluding to "The Unknown God."
      him declare--announce.
      I unto you--This is like none of his previous discourses, save that to the idolaters of Lycaonia (Ac 14:15-17). His subject is not, as in the synagogues, the Messiahship of Jesus, but THE LIVING GOD, in opposition to the materialistic and pantheistic polytheism of Greece, which subverted all true religion. Nor does he come with speculation on this profound subject--of which they had had enough from others--but an authoritative "announcement" of Him after whom they were groping not giving Him any name, however, nor even naming the Saviour Himself but unfolding the true character of both as they were able to receive it.

JFB.