Complex sentence: Athanasius of Alexandria proposed a theory of the atonement which similarly states that sin leads to death, and that God warned Adam about this, and so, to remain consistent with Himself must have Jesus die for sins, or have humankind die.
Split into simpler sentences: Athanasius of Alexandria proposed a theory of the atonement which similarly states that sin bears the consequence of death. In order to remain consistent with Himself must have Jesus die for sins, or have humankind die.