"Therefore this is what the Holy One of Israel says: 'Because you have rejected this message, relied on oppression and depended on deceit, this sin will become for you like a high wall, cracked and bulging, that collapses suddenly, in an instant. It will break in pieces like pottery, shattered so mercilessly that among its pieces not a fragment will be found for taking coals from a hearth or scooping water out of a cistern.'" NIV translation
Isaiah continues to recount the events that preceded the fall of the northern kingdom. We see, here, that the name of God that is used is "Holy One" and it was intentionally used to underscore the fact that God is separate and not a part of some alliance. Israel had rejected the promises of God and had put their hope in an alliance with Egypt but that alliance is described as a cracked and bulging wall.
This was a reminder to Jerusalem and is also one to us that God is not to be our "plan B" to be used in case all other efforts fail. He is to be set apart (holy) in our hearts as the One that is in control and has the power to protect us. If, like Israel, we choose to look to allies in this world to help us instead of trusting in the Lord, we will find out that they too are like a cracked wall.
The wall might make you feel good for a time but it really does not change the situation. We are also reminded of the fact that this attitude of alliances will be embraced in the last days and will make it possible for the antichrist to rise to power.
"This is what the Sovereign Lord, the Holy One of Israel, says: 'In repentance and rest is your salvation, in quietness and trust is your strength, but you would have none of it.'" NIV translation
God reminded the northern kingdom that He was in control and that all they had to do was to sit still and trust in Him. They did not do that, so they would face the consequences. The same type of thing applies to us as Christians. When things look their darkest, we can choose to wait on the Lord or we can search for a solution on our own.
If we choose to try to fix things on our own, then, God will allow us to do so but we will have to deal with the results from our bad decision.