'Maid servant' in the Bible

for it hath been written, that Abraham had two sons, one by the maid-servant, and one by the free-woman,

but he who is of the maid-servant, according to flesh hath been, and he who is of the free-woman, through the promise;

but what saith the Writing? 'Cast forth the maid-servant and her son, for the son of the maid-servant may not be heir with the son of the free-woman;'

then, brethren, we are not a maid-servant's children, but the free-woman's.