What is the doctrine of reincarnation?

The doctrine of Reincarnation is not an absolute law under which all spirits must pass, but spirits do believe that in the experiences of many spirits, reincarnation is a law of their progression.

Each spirit or soul born into planetary life has spiritual guardians who superintend its welfare and educate the soul by such means as they deem best. These spiritual guardians or angels differ in their methods and schools  of thought, for there is no absolute path on which all must walk alike.

Each maps out a different path by which the pilgrim souls will reach it.

The guardian angels watch over the soul-germ during all its childhood and youth, which lasts from the moment it first sees the light of individual consciousness until through repeated experiences and developments, it attains to such a degree of intellectual and moral consciousness that it stands on the same level, as its spiritual guardians―it in turn becomes the spiritual guardian of some newborn soul.

The soul-germ, in its first stage, is like any other seed in its minute size and powers. It is a spark of the Divine Essence and contains all that will constitute the perfected human soul. Its very essence is immortal and indestructible because it is the seed from that which is immortal and indestructible, but the soul-seed has to be sown into the corruptions of matter―first in its lower and then in its higher forms.

Each seed develops to its highest degree through successive spheres and experiences. Some schools of thought hold that the soul will progress more rapidly if it returns to material life to be born anew in a fresh form each time, and to live over again the experiences it has missed―or to expiate the wrongs done in a former incarnation. The spiritual children of this school of thought return to earth again, and for them each new lesson will have to be worked out in an earthly life. 

This experience is not necessarily the lot of all spirits. 

There are other schools who maintain that the spirit spheres contain means for the education of the soul quite as useful and expeditious. They will be made to live over in memory their past earth life and to expiate in the spirit the wrongs done in their earthly existences.

As each soul differs in its character or individuality, so each must be trained by a different method, else all would resemble each other so exactly that a monotonous sameness would result and there would be none of that variety and contrast, which give a charm to earthly life.