Isaiah 43:27-28. “Thy first father hath sinned, and thy teachers have transgressed against me. Therefore I have profaned the princes of the sanctuary, and have given Jacob to the curse, and Israel to reproaches.”
My immediate thought was that thy first father hath sinned was a reference to Adam. Much to my surprise, there is some disagreement among commentators on exactly who that “first father” was. Some believe it was Abraham, others that it was Jacob, who was the clear seminal head of the Israelite nation. Some believe it was Urijah, who was high priest during the time of Ahaz. Some believe it is a reference to all the forefathers collectively, and that belief would tie in with the idea of the ongoing persistence in Israel’s sin in turning away from God and worshiping idols.
My Dake’s Study Bible has this to say:
This could not refer to Adam who was the father of the whole race, not of the Jews only. It could only refer to Abraham whom the Jews consider the father of the race (Matt. 3:9; John 8: 33-39). It would have been out of order to refer to Adam’s sin in this case, for the prediction concerned the destruction of the nation of Israel, their city and temple (v. 28). To say that Abraham was always righteous and faithful to God and could not be referred to as a sinner is erroneous.
What follows is a list of seven sins of Abraham. It’s an interesting study, but one I won’t reproduce here.
I think the important point here is that the sin of Israel started at the very beginning, and was perpetuated by the leaders (teachers) down through the generations. The primary sin of the nation was to forsake the worship of the Holy One of Israel and turn to the idols of the nations surrounding them. It was for this sin that God says He will turn Israel over to reproach.