I think that one must consider the several unique perspectives of Calvary in order to get a full orbed view of the Divine Work of God.From God's perspective Calvary has always been a "finished" work. "The Lamb slain from the foundation of the world." Calvary in the Logos was predetermined, prearranged, premeditated and preemptive in the Mind of God. It was another one of those actions that only God can speak of as "things that are not (as to time) as though they were (already done in the plan of God as to His Will)." From man's perspective it was an unfolding drama of the Work of God. The past, present and future all had their play in the events. Man, as he is propelled along by time, was either touched by Calvary as to history or as to reality or as to a future event. The arms of the cross reach into all tenses. "In the fullness of time" is from man's perspective.
The Acts 2:38 message is the only way whereby Calvary becomes a living reality.Therefore the Cross is and always has been a "done deal" but was also a prophetic event of the future while it was also a "current event" and it was also a "historical" event. Calvary though historical is still contemporary because it IS; Calvary is an ever flowing power that keeps pace with every generation and never diminishes nor wanes. It is a living force brought about by the death of a Sinless Sacrifice. Calvary lives because death cannot relegate it to the pages of history only; Calvary defies time because it is eternal.Calvary, is-was-and shall be; it is an event before its time and of its time and without time. It is the stellar miracle of eternity. No wonder the preaching of the cross IS the power of God!
Why didn't God come immediately after the fall of Adam? That question has been asked many times. But scriptures such as contained in Gal 4 give a hint to the answer:Gal 4:4-5 But when the fulness of the time was come, God sent forth his Son, made of a woman, made under the law, To redeem them that were under the law, that we might receive the adoption of sons.Fulness of time: A time when all the prophecies would center on Him, and when there could be no doubt about their fulfillment. A time predicted so far before the event as to make it impossible to be mere human conjecture.Fulness of time: After the passage of thousands of years in order to prove to man that man could NEVER redeem himself. All such attempts miserably failed. Men were "without God and without hope" in their own devices. Only God could affect a salvation that would truly redeem!Fulness of time: The law could only testify to the failure of man and condemn him so Jesus Christ was "made under the law" to "redeem them that were under the law." He came into the system of condemnation--yet was not condemned Himself in oder to rescue those that were condemned. "Yet without sin." to die for those that were IN sin.