Revelation 3:7: “To the angel of the church in Philadelphia write: These are the words of him who is holy and true, who holds the key of David. What he opens no one can shut, and what he shuts no one can open.”
What if you are standing at the door of opportunity and it is closed? Who can open it? Can you do it yourself? Even if you can, will you be able to keep it open? I bet not. Why not let God do that for you? If he opens the door nobody will ever be able to close it. Just trust him.