The Oxford English Mini-dictionary defines a “plan” as a “method thought out in advance;” and, defines “to purpose” as “to intend to act.”
So the Lord of Hosts takes an oath. He swears, “As I have thought out in advance, so shall it come to pass, and as I have intended to act, so shall that action stand.”
Here the Lord declares His sovereignty. He tells us that He has a well thought out plan, a purpose filled with good intentions. All He thinks comes to pass. His method endures, is valid, is unmovable.
All we need do is trust Him.