There is one more consideration that will be the most controversial of all, and that is, John when saying the Whole World in its generical sense does also mean, the Whole World in Adam in the beginning, that is all nations Acts 17:26-27
26 And hath made of one blood all nations of men for to dwell on all the face of the earth, and hath determined the times before appointed, and the bounds of their habitation;
27 That they should seek the Lord, if haply they might feel after him, and find him, though he be not far from every one of us:
That's the basis for God's Promise to Abraham Gen 12:3
3 And I will bless them that bless thee, and curse him that curseth thee: and in thee shall all families of the earth be blessed.
Gen 17:5
5 Neither shall thy name any more be called Abram, but thy name shall be Abraham; for a father of many nations have I made thee.
Gal 3:8
And the scripture, foreseeing that God would justify the heathen through faith, preached before the gospel unto Abraham, saying, In thee shall all nations be blessed.
With this we understand that the Seed of Abraham was the only seed ever in Adam originally in the beginning at Creation. This is True because Christ was and is Abraham's Seed clearly Gal 3:16
16 Now to Abraham and his seed were the promises made. He saith not, And to seeds, as of many; but as of one, And to thy seed, which is Christ.
Now Adam was a figure of Christ or Him to come Rom 5:14
14 Nevertheless death reigned from Adam to Moses, even over them that had not sinned after the similitude of Adam's transgression, who is the figure of him that was to come [Christ in the flesh].
So for Adam to have the full analogical force of a type of Christ, Christ being Abraham's Seed, then it was Abraham's Seed represented in Adam in the beginning, and that was the World that Christ came to Identify with Heb 2:16
16 For verily he took not on him the nature of angels; but he took on him the seed of Abraham.
Now not included in Adam originally at the time of His Creation as the in the Image and Likeness of God Gen 1:26, not included was the seed of the serpent, which seed was created in Lucifer, this seed we hold to be the non elect outside of Christ, the Seed of Abraham. This vile seed is also part of mankind. This wicked seed was added to the seed of Abraham/Adam after the fall, in order that Adam would become though not its Federal Head as he was for the Elect, but he did become its Natural head, that is, He and Eve would be responsible for parenting naturally the devil's offspring due to their sin, that this evil seed would have an physical existence in the world to further carry out God's Eternal Purpose in Christ. These are men and women of the same physiological make up naturally as it is with the Holy Seed in Adam/ Abraham, however the depraved nature both seeds have by nature will remain permanently with the serpent's seed, whereas the Holy Seed, the Seed of Abraham/ Adam, a different Nature will be given them in another birth that is more excellent that their physical birth, a birth out of God, the New Birth, or Birth from above, but the seed of the serpent will never partake of this Birth. So Christ in this sense is the propitiation of the Whole World that was in Adam in the Beginning, which excludes the seed of the serpent. Gen 3:14-15
14 And the Lord God said unto the serpent[The devil, satan], Because thou hast done this, thou art cursed above all cattle, and above every beast of the field; upon thy belly shalt thou go, and dust shalt thou eat all the days of thy life:
15 And I will put enmity between thee and the woman, and between thy seed and her seed; it shall bruise thy head, and thou shalt bruise his heel.