John Calvin 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 to me. He gives two reasons for staying longer in Ephesus:

1. First, because an opportunity was given to him there for advancing the gospel.
2. And second, because due to the large number of adversaries there, his presence was especially needed.

“I will do much good by prolonging my stay here for a little while, and if I were absent, Satan would do much harm.” In the first clause, he uses a very common metaphor, employing the term door to mean an opportunity. For the Lord opened a way for him to advance the gospel. He calls this a great door because he could win many people. He calls it effectual because the Lord blessed his labor and made his doctrine effectual through the power of His Spirit.

We see, then, how this holy man sought Christ’s glory everywhere and did not select a place for his own convenience or pleasure. Instead, he simply considered this: where he might do the most good and serve his Lord with the most abundant fruit. In addition to this, he not only did not shrink back from hardships but willingly presented himself where he saw that he would have to struggle more intensely and with greater difficulty.

For the reason he remained was that many adversaries were near. And the better equipped he was to endure their assault, the more he needed to be prepared and resolute.