Albert Mackey

“ And hence, in Freemasonry, the same symbolism is communicated by the apron, which, because it is the first gift which the aspirant receives,--the first symbol in which he is instructed,--has been called the "badge of a mason." And most appropriately has it been so called; for, whatever may be the future advancement of the candidate in the "Royal Art," into whatever deeper arcana his devotion to the mystic institution or his thirst for knowledge may carry him, with the apron--his first investiture--he never parts. ”

“ The pure, unspotted lamb-skin apron is, then, in Masonry, symbolic of that perfection of body and purity of mind which are essential qualifications in all who would participate in its sacred mysteries. ”

“ We see, therefore, the propriety of adopting this color in the masonic system as a symbol of purity. This symbolism pervades the whole of the ritual, from the lowest to the highest degree, wherever white vestments or white decorations are used.
As to the material of the apron, this is imperatively required to be of lamb-skin. No other substance, such as linen, silk, or satin, could be substituted without entirely destroying the symbolism of the vestment. Now, the lamb has, as the ritual expresses it, "been, in all ages, deemed an emblem of innocence;" ”