For if the mighty deeds done in your midst had been done in ...
They are being condemned not for something they have done, but for something they have failed to do. God has done mighty deed but they have failed to see them.
Even in our own time how often do people look for God in the bad and not the good. When the hurricane hit New Orleans, how many were calling it punishment from God? Even in law an "act of God" is an event with and unforeseen cause ( eg. earthquake, tornado, flood). Nothing good is a legal act of God.
I think part of our problem is that if we see any human envolment we exclude the possibility that something is an act of God. We forget that as Christians we believe that the Holy Spirit that is God dwells in human beings and therefore can act in concert with human beings.
In the Gospel entire towns a are condemned for not recognizing the marvelous things God has done in their midst. When we stand before God do we want to face that same condemnation. This sin is easy to avoid. All we have to do is look around. We don't have to look far to see God's marvelous deeds done in our midst.