A.T. Robertson Commentary 1 Corinthians 16:9

"for a great door and effectual is opened unto me, and there are many adversaries." — 1 Corinthians 16:9 (ASV)

For a great and effectual door is opened unto me (θυρα γαρ μο ανεωιγεν μεγαλη κα ενεργης). Second perfect active indicative of ανοιγω, to open. Intransitive, stands wide open at last after his years there (Acts 20:31). A wide open door. What does he mean by ενεργης? It is a late word in the Koine. In the papyri a medical receipt has it for "tolerably strong." The form ενεργος in the papyri is used of a mill "in working order," of "tilled land," and of "wrought iron." In the N.T. it occurs in Phm 1:6; Hebrews 4:12 of "the word of God" as "ενεργης" (powerful). Paul means that he has at least a great opportunity for work in Ephesus.

And there are many adversaries (κα αντικειμενο πολλο). "And many are lying opposed to me," lined up against me. These Paul mentions as a reason for staying in, not for leaving, Ephesus. Read Ac 19 and see the opposition from Jews and Gentiles with the explosion under the lead of Demetrius. And yet Paul suddenly leaves. He hints of much of which we should like to know more (1 Corinthians 15:32; 2 Corinthians 1:8f.).