Isaiah wrote to a people who had forgotten why God had called them. Israel had come to think God established His covenant with them, soley for their enjoyment of relationship with Him. But Isaiah called the people of Israel to remember their purpose: 'To be a blessing to all peoples' welcoming them into a relationship with God, too!
Isaiah 54 is a call to the children of Israel, and to us, to stretch and change our lives to filfill His purpose and plan for us as His people. They, and we, are called to 'Expand, Lenthen, and Strenthen' our lives to make room and welcome those God is calling to Himself.