Were our sins laid upon Christ at the Cross, at Conception or at the Covenant?

The Heidelberger teaches

Question 37. What dost thou understand by the words, "He suffered"?

Answer. That He, all the time that He lived on earth, but especially at the end of His life,

[A] sustained in body and soul, the wrath of God against the sins of all mankind: that so by His passion, as the only
[B] propitiatory sacrifice, He might redeem our body and soul from everlasting damnation, and obtain for us the favor of God, righteousness and eternal life.