But no one has to be completely sure just to investigate a religious vocation. If religious life has been a gut question over a period of time, it's worth exploring.
If others have agreed that a religious vocation seems fitting -- or worth investigating all the more reason to step forth. By entering a formation program, people commit to a particular time of exploration, not to the end result of religious life. The degree of certainty becomes more important as people move through and near the end of formation.
It is less important in the beginning.