I was a young teenager the first time I heard the word, "vocational." They used it in High School to refer to one of the tracks a student could enroll in. One was a college preparatory track, and the other was a vocational track that prepared you for the work force after you got out of High School. The vocational program included advanced training in auto-mechanics, welding, auto-body, upholstery, telecommunications, drafting, or food service.
At the time, "vocation" was a word with no religious meaning behind it at all. It simply dealt with your job, career, or profession.
Then I learned that the word, "vocation" came from the Latin word, "vocare" which means "calling." I also learned that it was strictly a religious word. Those who had a "vocation" or "calling" were those who were called into professional ministry. The clergy of the church were those with the vocation/calling. Everyone else did not have a calling. They simply worked to support themselves and to support those who did have a calling.
So, a very religious word became completely stripped of its religious meaning into someone complete "secular."
However, during the reformation, there were those who reclaimed the idea of "vocation" or "calling" for its true intention in scripture. They taught that every single Christian had a calling and it was not limited to professional ministry. Whether one was a brick-layer, a cobbler, a farmer, a seamstress, or had some other trade, their profession was also a calling from God. Humans were created by God to be rulers and caretakers of God's world. They were to multiply and fill the earth. As they did so and began to study the world, harnessing its energies for good, establishing institutions to govern, educate, build, and engage in creative activities, they were fulfilling God's cultural mandate given to mankind at the creation. Some reformers referred to humans as being "masks of God," because their created and ruling activities mirrored the type of activity God engages in.
However, sin came into the world and corrupted the image of God in man. He needed redemption and restoration of his very nature. God promised to send his Spirit who would renew the heart of man and transform him into the image of the Lord which had been corrupted by sin. This restoration is a necessary step to recover and be able to accomplish the original cultural mandate God gave in Genesis. Mankind could not accomplish this without redemption and restoration.
Now, when we are called by God, we are transformed and reformed. This inner change may not mean we change occupations, but it definitely means our purpose changes. All our work is in service to God and his purposes. Whether we fix things, lead people, teach, heal, build, etc, we are part of the body of Christ. He works through us to transform the world. In order words, we promote the reign of God in two indispensable ways. We do so through the Great Commission, sharing the Gospel as it transforms people, which in turn transforms our occupations for God's purposes, which fulfills the Cultural Mandate. It is one of the ways we are salt and light in the world.