White muslim is perfect for Cathedral Windows quilts. You can choose just about any focus fabrics for the "windows," and it'll look great with the white. Traditional Cathedral Windows use a lot of fabric. You'll be glad you have so much!
FWIW, before I made my first CW quilt, I made a small sample with some leftover white muslin and some scraps of Christmas fabric. Because I was just trying out the technique, I didn't bother changing the thread in my machine. Wish I had, cuz the resulting sample was so gosh darned cute - except for the tan thread. I'd have bought more muslin and kept going!