God is sovereign and He gives us freedom. So rather than having one direction that we must follow or else we are “out of His will” (a concept never found in Scripture, outside of specific sin related issues), He entrusts us with “talents” and stewards us with their use (as in the parable of the talents in Matthew 25). It is our responsibility to make choices that honor Him and to invest His resources well (for His glory). Along the way, we encounter opportunities to use those resources.
We might refer to those opportunities as open doors, and because He is sovereign and in control, there is no problem recognizing that God has opened a door (notice how Paul does that in 1 Corinthians 16:9), recognizing that we still have to assess obstacles, etc., and use His (provided) wisdom (as in James 1:5) to decide whether or not it is the best use of His resources to go through that door. There is nothing mystical about these open and shut doors – if a door opens, we know that it couldn’t open if God didn’t (at least) allow it to be opened. If a door closes, then we know that it couldn’t have closed without His OK. Consequently, it is wise to be constantly looking for the right doors to go through in order to maximize His resources, and to be faithful with the stewardships He provides.