Why does it say "v'Amarnu"? The ones speaking now will not say this, for later they will say "the Mizbe'ach that our fathers made"!
Radak: It is as if says "v'Amru"; our children will be in place of us. We find "v'Osanu Hotzi mi'Sham" (Devarim 6:23; it refers to their fathers).
Who is 'Avoseinu' who made the Mizbe'ach?
Malbim: It is all of Yisrael. The others did not protest; this shows that they consented.