That the Father sent the Son to be the saviour of the world and that John declares of him ‘Behold the Lamb of God which taketh away the sin of the world’, does not suggest, in any way, a ‘Christian Universalism’, that is to say the notion that, automatically, all humankind shall be saved irrespective of their behaviour, irrespective of faith or irrespective of the purposes of God in salvation.
In order that there should be a world to come, at all, necessitated the coming of Christ and the death of Christ. It is clear from what the scriptures teach regarding restoration (unhelpfully translated ‘reconciliation’) that salvation is effected by the death of Christ and by union with Him, under his Headship, in a righteous restoration out from under the headship of Adam and out of the former human state in Adam.
Part of this restoration is the fact that Jesus Christ is the Lamb of God who taketh away the ‘sin of the world’ which does not refer to the specific actions of individuals (which is dealt with in other parts of the doctrine of Christ) but refers to the entry of sin into the world from the beginning, in Adam.
Being ‘made’ sin, or being effected sin (some translate this as ‘being made a sin offering’ but I suggest that that falls short of the full concept) sin, itself, was condemned within the humanity of Jesus Christ - a clean humanity - and sin was eradicated in the sight of God, righteously.
No individual is condemned, personally, for the liability of flesh and blood, or for the propensities of created humanity, or for the failure of man in the flesh, or for the transgression of the head of humanity, Adam. This liability and this sin is taken away by the Saviour and borne by him in his death.
That removal, righteously, is necessary for there to be a world at all. That removal was necessary even to grant a continuance to humanity after the Flood. The judgment on the world, by water, was to end all flesh because of the consequences of sin and because of the multiplication of evil on earth to the extent it could not be tolerated any further.
Eight persons were preserved and that preservation was in order for the purposes of God to be fulfilled, despite the transgression, : a continuance being granted, due to a foreseen sacrifice (by Christ) that gave a righteous basis for the extension of the world, in time, for those divine purposes to progress.
Else, in righteousness, there could be no world at all. The world would have ended at the time of the Flood.
He is the ‘Saviour of the world’ in that humanity is granted existence, time, forbearance and the testimony of the gospel (whether by figure and ritual under the old covenant, or whether by full revelation under the New Testament) that they might be saved.
But if there is unbelief and rejection then that is the responsibility of each individual. And the consequence is to their own account, personally.
The further notion that the sufferings of Christ (within his body, prior to his death) were effective for the sins of all humankind (but only if certain individuals took advantage and added their own effectiveness) is nowhere expressed in scripture and is, in effect, salvation by works.