The department shall not approve the articles until at least 15 lending institutions authorized to do business in this state have agreed in writing to become members of said corporation; and said written agreement shall be filed with the department with the articles, and the filing of same shall be a condition precedent to the approval of the articles by the department. Whenever the articles shall have been filed in the office of the Secretary of State and approved by the department and all filing fees and taxes prescribed by law have been paid, the subscribers, their successors, and assigns shall constitute a corporation. Said corporation shall not be authorized to commence business until its articles are approved by the department. Upon such approval by the department, authorized stock of the corporation may thereafter be issued.