When that question was asked of you during the work of the First Degree, did it register as being of great importance? To most candidates it doesn't.
Usually it appears to be just a question that occurs in the process of instruction, and the answer seems to be very simple, of course, but as the student of Masonry progresses, he discovers that even the simplest of thoughts in Masonry have deep meanings attached to them. Very little occurs in the work of the Craft that is of small significance.
All of us have noted certain Brethren in our own and in other Lodges who seen particularly apt to learn and express themselves Masonically. These men are sure to be found at the head of Lodge activities. They are moving spirits. They know the work, and they know how to get things done. Moreover, they appear eager to advance in the interest of their Lodges. They think up things to do, or rather observe things that need doing. Sometimes, their energy even grates a little on other Brethren. Once in a while, criticism may be heard, the burden of which is likely to be that these Brethren are a trifle too ambitious.
One knows himself to be a Mason when he begins to view truth, honor, justice, and charity not as abstract principles, but as practical everyday virtues which he must practice and exemplify in all of his relationships with his fellow men. When within him burns and intense desire to protect and advance the welfare of his Brethren and with a constant care, that no harm or injury comes to them through acts of his, a man may know he is developing the Masonic spirit.
Such a thing may occur once in a while to be sure, but not often. As a rule the active Brother is one who knows he is a Mason, why he is one, and how he got that way. To such a Brother the question of how he knows he is a Mason is a sort of challenge. He knows very well the simple answer in the ritual isn't all of it by any means. It is only the beginning. He knows the ceremony whereby he was brought to light was merely the lighting of the lamp. By it he was able to enter the Lodge, to greet the Brethren Masonically, but so far as actual work was concerned he was verily an Entered Apprentice.
The light illuminated his own character and revealed to him obligations owing to his fellowmen. It was then he began to understand knowing a few of the secrets of the Craft is now what makes him a Mason. It is true that many good men feel these same things without being Masons. But any understanding Mason knows such men would accomplish more and find greater joy in their service, if added to their natural goodwill, they had the encouragement, fellowship, and inspiration of a great fraternity wholly dedicated to the same high aspirations.
You know yourself to be a Mason first by understanding the eternal principles upon which the Craft is founded, and then by feeling in your own heart a clear response to their call. When, gradually, the new Brother learns of the glorious history of the Fraternity and begins to feel kinship with the legions who went this way before him; when in the events of the past he is able to discern the fine handcraft of brothers and fellows, tracing out their beneficent influence in the progress of mankind; then he knows himself to be a Mason in spirit and in truth.
The place to look for evidence that you are a Mason is not in the head, where memory of rituals, grips, signs, and tokens may be retained, but in the heart from whence will flow the healing streams of brotherly love, generosity, kindness, patience, and goodwill. If those are found you will know your Masonic fellowship has borne good fruit and you are in fact a Mason.
Source: How Do You Know Yourself to Be a Mason?
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