The origin of the word "Pillar" is "Pilier" (French). In the sense that it is being used here, it refers to the senior officers of the Order of St. John who were "piliers" or heads of the "Langues" or "Tongues" - some more difficult words for us moderns to get our heads around because they did not have quite the meaning in those times that we give them today.

When the Order was in Rhodes, it organised itself in a way that has lasted for centuries. Those members who came from the same regions, tended to stick together and sowere grouped in the "Langues" or "Tongues". I imagine they were so called because they all spoke the same language or tongue. There were eight of them: Provence, Auvergne, France, Aragon, Germany, Castile, England and Italy.

The Piliers" (Pillars), together with the Bishop of the Order, the Prior, the Bailiffs and Senior Knights formed the Council or advisory body to the Grand Master. The Piliers were each assigned a special function, thus the Pilier of Italy was the Admiral (as the BC member has rightly said); the Pilier of France was the Hospitaller; the Pilier of Provence was the Grand Commander and the Pilier of England was the Turcopilier or commander of the light cavalry.

According to the book "The Orders of St. John", there were eight   Langues, one for each point of the cross, and each represented by a Conventual Bailiff or Pillier.  Provence,(the Grand Commander), Auvergne (the Marshal), France (the Hospitaller), Italy (the Admiral), Aragon (the Drapier, later Grand Conservator), England (the Turcopilier), Germany (the Grand Bailiff) and Castille-Portugal (the Grand Chancellor).  Each of the eight Langues created their own headquarters called an Auberge, in which they lived with their Pillier. 

History buffs may appreciate that this is a scandalous story about the Pilier of Castile who was the Grand Chancellor. When the Order was fighting it out with the Turks, a spy was discovered passing information to the enemy. He said that he had been put up to it by no other than Andrea d’Amaral, the Pilier of Castile. Andrea was an extremely unpopular man and was embittered because he believed that he should have been made Grand Master instead of Philip de L’Isle Adam. He was accused of treachery but although tortured before being executed, he never did confess so the truth will never be known.