The question is posed incorrectly. It should ask if the sacrifice of the Son is sufficient to cover the sins of all humanity.
The answer is yes. It is sufficient to cover the sins of all humanity. Why doesn't everyone come to the Lord?
Because they choose not to, given that 2. - - Christ provided a universal opportunity by dying for all men and for every man such that ALL those who turn to him as true repentant believers are saved. (See Arminian construct above)
Where have we said that the atonement is not sufficient to cover all of the sins of the whole world, without exception?
We have specifically affirmed that fact. However, the question that you duck is this: Does the atonement effectively cover all the sins of the whole world, without exception? Why not? Because of unbelief. Isn't unbelief a sin?