1)
What should they contemplate?
1.
Radak: This rebuke, and repent from their evil deeds.
2.
Malbim: Remember Elokim, who knows hidden things!
2)
Why did He say "Shochechei Eloka"?
1.
Radak: Until now, you forgot Elokim.
2.
Malbim: Had you not forgotten Elokim, you would fear Hashem who knows your deeds, and not deceive with flattery and pretensions via bringing Korbanos and Sheker 1 about your brothers.
1
Pretending to be angry about their Aveiros. (PF)
3)
Why did He say "Pen Etrof v'Ein Matzil"?
1.
Radak: I will not delay anger longer; the day of judgment came. Also in Mal'achi it says "Pen Avo v'Hikeisi Es ha'Aretz Cherem" (3:24).