1)
What is the matter that we spoke?
1.
Rashi: It is the Bris we made. Malbim - therefore, do not endanger yourself to see me.
2)
Why did he say "Hashem is between me and you"?
1.
Rashi: He is a witness between me and you.
2.
Get Pashut (129:121): A Yud was added to David's name and a Hei to Yonason's name, to hint that they put Hashem's name Yud-Kei between them, like it is between man and wife (refer to 18:1:1:1*).
3)
What is forever?
1.
Radak: [Malbim - Hashem is a witness about] the Bris between us.
