An interlocutor once asked me,
"Dear Prof. Foster,
I'd like to know when the concept of immortal soul was adopted by early christians and which Church-Father rejected it."
The concept of the immortal soul appears rather early in the second century CE writings of pre-Nicene writers. One of the most notable delineations of this teaching is found in Letter to Diognetus 6. There, we read:
"To sum up all in one word-what the soul is in the body, that are Christians in the world. The soul is dispersed through all the members of the body, and Christians are scattered through all the cities of the world. The soul dwells in the body, yet is not of the body; and Christians dwell in the world, yet are not of the world. The invisible soul is guarded by the visible body, and Christians are known indeed to be in the world, but their godliness remains invisible."
Justin Martyr, Melito, Athenagoras, Methodius, Irenaeus, Origen, Clement of Alexandria, Lactantius and Tertullian all affirmed the immortality of the soul (see Divinae institutiones 7.20ff for statements from Lactantius). Tertullian thinks that the soul is corporeal, though not constituted of gross matter or flesh. See his work De Anima.
It seems that the teaching of the immortal soul was consistently believed and taught in the early church, though an early Assyrian apologist named Tatian, posited an interesting theory in his Oration to the Greeks 13, namely, that the soul "is not in itself immortal . . . but mortal. Yet it is possible for it not to die."
In other words, the human soul (according to Tatian) is not inherently immortal. Eternal or everlasting life is only possible, provided one has a good relationship with God, who can cause the soul to subsist forever by investing it with the gift of immortality. It is no wonder that Jaroslav Pelikan (The Christian Tradition 1:30) speaks of the immortal soul teaching as a "standard element in [early] Christian teaching" since the pre-Nicene and post-Nicene church uniformly affirmed this doctrine.