All eternal souls must have pre-existence and post-existence, otherwise they wouldn't be 'eternal'! Most Christians and Muslims believe that "When you die, you go to heaven for eternity". But that's not the definition of eternity - that's the definition of perpetuity. When you pick a point74 (in time47) and continue forever74 - either forward or back - that's perpetual. When one includes all of time - past, present, future, the beginning, the end, a constant (the speed of light), etc. - that's eternal.
The early Church Fathers77 were split on whether to promote reincarnation. Clement and Origen of Alexandria taught the transmigration of the soul - metempsychosis - and taught that the Christ77 taught it, "He who hath ears to hear, let them hear".
References
Reincarnation - The Missing Link In Christianity by Elizabeth Clare Prophet (Summit University Press, 1997)
Reincarnation: The Phoenix Fire Mystery
Famous Direct Reincarnations by Brad Watson (projected 2013-14)