A disciple is one who above all else, is pledged to do three things:—
a. To serve humanity.
b. To co-operate with the plan of the Great Ones as he sees it and as best he may.
c. To develop the powers of the Ego, to expand his consciousness until he can function on the three planes in the three worlds, and in the causal body, and to follow the guidance of the higher self and not the dictates of his three-fold lower manifestation.
A disciple is one who is beginning to comprehend group work, and to change his centre of activity from himself (as the pivot around which everything revolves) to the group centre.
A disciple is one who realises simultaneously the relative insignificance of each unit of consciousness, and also its vast importance. His sense of proportion is adjusted, and he sees things as they are; he sees people as they are; he sees himself as he inherently is and seeks then to become that which he is.
A disciple realises the life or force side of nature, and to him the form makes no appeal. He works with force and through forces; he recognises himself as a force centre within a greater force centre, and his is the responsibility  of directing the energy which may pour through him into channels through which the group can be benefited.
The disciple knows himself to be—to a greater or less degree—an outpost of the Master's consciousness, viewing the Master in a two-fold sense:—
a. As his own egoic consciousness.
b. As the centre of his group; the force animating the units of the group and binding them into a homogeneous whole.
A disciple is one who is transferring his consciousness out of the personal into the impersonal, and during the transition stage much of difficulty and of suffering is necessarily endured. These difficulties arise from various causes:—
a. The disciple's lower self, which rebels at being transmuted.
b. A man's immediate group, friends, or family, who rebel at his growing impersonality. They do not like to be acknowledged as one with him on the life side, and yet separate from him where desires and interests lie. Yet the law holds good, and only in the essential life of the soul can true unity be cognized. In the discovery as to what is form lies much of sorrow for the disciple, but the road leads to perfect union eventually.
The disciple is one who realises his responsibility to all units who come under his influence,—a responsibility of co-operating with the plan of evolution as it exists for them, and thus to expand their consciousness and teach them the difference between the real and the unreal, between life and form. This he does most easily by a demonstration in his own life as to his goal, his object, and his centre of consciousness.
Alice A. Bailey, Initiation Human and Solar. pp. 71 - 72