Why did Jesus take all of mankind’s sin?
He took away their past sins.
Not the sins committed wilfully after gaining knowledge of the truth through the gospel.
This is what God thinks of those people who sin presumptuously:
Numbers 15:30-31
[30] But the soul that doeth ought presumptuously, whether he be born in the land, or a stranger, the same reproacheth the Lord; and that soul shall be cut off from among his people. [31] Because he hath
despised the word of the Lord, and hath broken his commandment, that soul shall utterly be cut off; his iniquity shall be upon him.