Heart of the Gospel
Firstly, God Speaks through His Holy Scripture even “secret things”:
“so that it might be fulfilled that having been spoken by the prophet, saying: “I will open My mouth in parables; I will utter things hidden from the foundation of the world.” (Matthew 13:35)
(i) What is God’s Will toward “all things”?
“and through Him to reconcile all things to Himself, having made peace through the blood of His cross; through Him, I say, whether things on earth or things in the heavens.” (Colossians 1:20)
*Note: Only things at “enemies with God” need “Reconciliation”. Universal Reconciliation is spoken of here using “different words” since such a Reconciling Act is applied towards “all things” (Context) and “not” just towards believing or Christian things only (“a primary or special subset” closest to the Heart of God).
(ii) Does the “all things” include “His enemies”?
“For He must reign until He has put all His enemies under His feet.” (1 Corinthians 15:25)
(iii) What happens next? Does the “subjection” of “His enemies” to Him imply “a negative or a positive End” for all?
“When all things are subjected to Him, then the Son Himself also will be subjected to the One who subjected all things to Him, so that God may be all in all.” (1 Corinthians 15:28)
Hence, the Heart of the Gospel includes the Message that “God Will be All in All” (including the “enemies subjected to Him” just a few verses prior) as it is written clearly above (even in Context).
(iv) Will all be saved by Him at the “same” time?
No, as each one is “Saved by Him” according to its own “appointed times” (“in its order):
“But each in his own order: Christ the first fruits, after that those who are Christ’s at His coming,” (1 Corinthians 15:23)
(v) How do we know that this is “The End”? (Lord Jesus Christ fulfilling the Will of God Perfectly)
“then comes the end, when He hands over the kingdom to the God and Father, when He has abolished all rule and all authority and power.” (1 Corinthians 15:24)
*This is “not” a manipulation of Holy Scripture because the couplet of verses above in 1 Corinthians 15 Speak of “This End” where God finally becomes “All in All”.
(vi) Did “all” prophets who were sent by God “speak” (“not” necessarily written down) about this “Universal Hope of Restoration” in some way?
“whom indeed it behooves heaven to receive until the times of restoration of all things, of which God spoke by the mouth of His holy prophets from the age.” (Acts 3:21)
(vii) Conclusion?
Thus, the “Heart of the Gospel” speaks or reveals the “Heart of Christ (God)” to “Reconcile & Restore All things” to Himself thereby causing the “Whole World to be Saved” fulfilling the Immutable Will of God where “God becomes All in All” in “The End”.
“We have seen and testify that the Father has sent the Son to be the Savior of the world.” (1 John 4:14)
*We = All apostles & earliest believers of the First Church of Christ the Lord.
Blessed Saviour of the World.