Whether or not Hiram was a true believer, I think it can be safely assumed that he was partial to God's annointed leaders and had sincere respect toward the true religion. God can work in the hearts of kings (Prov. 21.1) for the good of his people, and has done so on many occasions (Cyrus, Nebuchadnezzar, Ahasuerus, Darius, etc.). I think Hiram's particular role in the redemptive history of God's people was a temporal fulfillment of both Isa. 60.10 and Ps. 72.11. The reign of Solomon, in particular, was a golden age characterized by the honor and peace which he had among the nations, which is a type of the mediatorial reign of Christ. Hiram had affinity for David and Solomon, and acknowledged the goodness of their God, and provided assistance, and thus, to a certain extent did good (Ps. 2.10).
Matthew Henry says (re 2 Chron. 2.3):
It should seem that Huram was not only very well affected to the Jewish nation, and well pleased with their prosperity, but that he was proselyted to the Jewish religion, and worshipped Jehovah, the God of Israel (who was now known by that name to the neighbouring nations), as the God that made heaven and earth, and as the fountain of power as well as being; for he sets up kings.
He rejoiced greatly; being an ingenuous prince, a lover of excellency, and a faithful friend to David and to his house. Blessed be the Lord; for though it be not probable that he was a sincere proselyte, because he did not endeavour the instruction of his people, and the extirpation of their gross idolatry, which by God's blessing and Solomon's help he might easily have effected; yet he had sufficient information concerning the nature and excellency of the God of Israel, and had honourable thoughts of him, as also divers other heathens had, 1 Sam 4:8; Dan 6:16; 2 Macc. 3:3.
