Adam Clarke on 1 Corinthians 1:9

A great door and effectual is opened - Θυοα γαρ μοι ανεωγε μεγαλη και ενεργης· A great and energetic door is opened to me; that is, God has made a grand opening to me in those parts, which I perceive will require much labor; and besides, I shall have many adversaries to oppose me. So Bp. Pearce understands the words ενεργης, not as signifying effectual, but as implying full of labor. Door often signifies occasion or opportunity; but here, the apostle may allude to the throwing open of the great doors of the Circus Maximus before the chariot races began; and the many adversaries may refer to the numerous competitors in those races.

God gave him a grand opportunity to preach the Gospel; but he was not to expect that either Satan or wicked men would leave him unmolested.