Which words come from Hashem's mouth?
Rashi: Those that He informs you via Nevi'im.
Radak: His promise to take you out of Galus; the Nevi'im said so from His mouth.
Are Hashem's words guaranteed to help?
Rashi: If you will heed them, they will benefit you.
Malbim: No. One who makes a Shali'ach for his own benefit, e.g. to pay a craftsman to make a Kli, if he will not make it, the Shali'ach returns empty-handed. If the Shali'ach is to benefit another, e.g. give money to my son if he will learn healing, he will not return empty. If the son did not learn, he will return the money! Hashem's word (good influence) returns empty if [the recipient] "Asah Asher Chafatzti." If he did not do so, he is not prepared to receive the good, and the good will return to me.
Why does it say "v'Hitzli'ach"?
Radak: No mishap will occur that will nullify [even] part. Everything will happen like I said.
Malbim: If the recipient succeeded (prepared himself to receive My influence), he will receive it, and My Shali'ach will return empty, like I explained (refer to 11:11:2:2).
