Complex sentence: On completing their studies, candidates were formerly "licensed to preach" by their home presbytery and became a probationer, serving a 12 or 18 month full-time probationary period in a parish.
Split into simpler sentences: On completing their studies, candidates are "licensed to preach" by their home presbytery. The candidate then becomes a probationer, serving a one-year full-time probationary period in a parish.