Albert Barnes Commentary Isaiah 36:15

"neither let Hezekiah make you trust in Jehovah, saying, Jehovah will surely deliver us; this city shall not be given into the hand of the king of Assyria." — Isaiah 36:15 (ASV)

Make you trust in the Lord - Rabshakeh knew that Hezekiah was professedly devoted to Yahweh, and that he would endeavor to induce the people to trust in Him. The Jews had now no other refuge but God, and as long as they put their confidence there, even Rabshakeh knew that it was hazardous to attempt to take and destroy their city.

It was his policy, therefore, first to endeavor to undermine their reliance on God, before he could have any hope of success. The enemies of God’s people cannot succeed in their designs against them until they can unsettle their confidence in Him.