Jamieson, Fausset and Brown on Isaiah 54:2

2. (Is 49:19, 20; Jr 31:31-36, 38, 39). Thy children shall be so many that thy borders must be extended to contain them.

curtains--the cloth forming the covering of the tent.

spare not--give abundantly the means for the enlargement of the Church (2Co 9:5-7).

cords ... stakes--The more the tent is enlarged by lengthening the cords by which the cloth covering is fastened to the ground, the more the stakes supporting the tent need to be strengthened; the Church is not merely to seek new converts, but to strengthen those she has in the faith. The image is appropriate, as the tabernacle was the symbol of the old Israelitish Church (see on Is 33:20).