All people are born spiritually dead in sin and under the death penalty for the broken law of God.
Indeed this is the belief to which Augustine eventually devolved in the course of his extended debate with Pelagius. I will state the understanding of the early church relative to "life" and "death" below, that of Athanasius in particular.
To say that we are not negates the purpose of Christ even coming and saving us from sin and said broken law.
This is in no way the case. Christ came to save us from such tyrants as sin, death, the devil, and the law, perfect though it was; from none of which could humans save themselves, death being their final demise. There is no compulsion to adopt Augustine's view in order to surmise a purpose for the absolute necessity of a Savior. If humanity were to gain victory over the tyrants, with God's adoption as sons and daughters, inclusive of life everlasting, being the ultimate purpose of their existence (see Eph 1.4-9), Christ must NECESSARILY have come.
The heart of man is deceitful above all things and desperately wicked - don't listen to it.
Good advise but beside the point.
Early Christians, Athanasius in particular, understood "life" to be an everlasting state of "becoming," the ultimate purpose of which becoming like God. Since God was infinite and eternal, those who experience life could and would
forever be
becoming more and more like God, in fellowship and union with Him.
With that as their understanding and definition of life, "death" was understood to be its negation: death was therefore the
unbecoming of life, its finale being an absolute return to
nothingness, out of which life and all else had been created.
Hence their understanding: Adam was created on a trajectory of becoming more and more like God as he matured in communion with God. Precisely at his fall, that trajectory changed. With sin came death, and it was inescapable. Separate a Savior, Adam was now on a course which would return him to that from which he had been created. This was true of his progeny as well ("All go to the same place; all come from dust, and to dust all return" Ecc 3.20; see also Gen 3.19). After the fall, "becoming" was no more. As a result of sin, all humans could do was die.
In Scripture and to the early church, death meant "a return to nothingness" ~ and nothing more. Christians had no concept of a spirit which could be separated from the soul ~ i.e., the entirety of a person ~ and somehow be dead while the rest of the person lived and functioned as a human being. Death meant the end of the entire person: spirit, mind, and body! This dichotomous view of person-hood was introduced by Augustine approx. 400 years after the cross, and it came as a result his pagan background, where he was heavily influenced by Manichaeism and afterward by the neo-Platonism of Plotinus. In Scripture and the writings of the early church, there is no mention of "spiritual" death. There is only life and death. To live is to grow in communion with God, in Jesus Christ, through the Holy Spirit. And to die is to return to that from which we were created: non-existence, no-thingness.
Have a blessed day.