Isaac is chosen over Ishmael

Gen 25:5-6 And Abraham gave all that he had unto Isaac. 6 But unto the sons of the concubines, which Abraham had, Abraham gave gifts, and sent them away from Isaac his son, while he yet lived, eastward, unto the east country.

Isaac is chosen by God

Heb 11:17-18 By faith Abraham, when he was tried, offered up Isaac: and he that had received the promises offered up his only begotten son. 18 Of whom it was said, That in Isaac shall thy seed be called.

Isaac represents being born not after the flesh, but after the spirit

Gal 4:28-29 Now we, brethren, as Isaac was, are the children of promise. 29 But as then he that was born after the flesh persecuted him that was born after the Spirit, even so it is now.