Gideontheson of Joashhathdone this thing - They fixed on him the more readily because they knew he had not joined with them in their idolatrous worship.
Matthew Henry
Concise Bible Commentary
See the power of God's grace, that he could raise up a reformer; and the kindness of his grace, that he would raise up a deliverer, out of the family of a leader in idolatry. Gideon must not think it enough not to worship at that altar; he must throw it down, and offer sacrifice on another. It was needful he should make peace with God, before he made war on Midian. Till sin be pardoned through the great Sacrifice, no good is to be expected. God, who has all hearts in his hands, influenced Joash to appear for his son against the advocates for Baal, though he had joined formerly in the worship of Baal. Let us do our duty, and trust God with our safety. Here is a challenge to Baal, to do either good or evil; the result convinced his worshippers of their folly, in praying to one to help them that could not avenge himself.
SDA Bible Commentary (limited)
Volume 1-7
29. Who hath done this thing? We do not know who betrayed his secret. Suspicion might naturally fall upon Gideon, whose tendencies toward the true worship of God may have been well known.