Read Isaiah 8.
The LORD provided more confirmation of the invasion of the northern kingdom of Israel by the Assyrians. He gave Isaiah three things to substantiate this prophecy. It would be in writing. It would be witnessed. Isaiah and his wife would have a son (with the longest name in the Bible). The timing of the weaning of the child would coincide with the God's word being fulfilled.
Meanwhile, the southern kingdom of Judah had no cause for fear. The Assyrians would put enormous pressure upon them but they would not be able to conquer Judah. Why?
Because God was with them. (vv.8-10)
His presence would not keep them from experiencing the stress. Rather, He assured them that He would bring them through in victory.
Because the LORD of hosts is holy. (v.13a)
God was sovereignly overseeing these events. All of heaven's armies (hosts) were at the ready to protect Judah.
So, what were the people to do?
1. Instead of fearing an invading army, they were to fear God and dread disobeying Him. (v.13b)
2. Instead of turning to satanically inspired alternatives, they should pray to the LORD alone. (v.19)
The people had been influenced by the surrounding pagan cultures to use mediums, witchcraft, even praying to dead people for wisdom and help. Paul reminds us in 1 Timothy 2:5 that there is one God and one mediator between God and men, the man Christ Jesus.
3. Instead of looking to the philosophies and writing of humans, they should be reading and heeding the written Word of God. (v.20)