John 8:35

And the bondservant abideth not in the house for ever: the son abideth for ever.

This is a reminder to sinners all that the penalty of sin is death. Bondservants of sin that men are, their days in the house of flesh are limited. This introduced another element of the bondage from which the truth makes free, the bondage of our mortality. Hendriksen saw an additional implication:

The old dispensation with its special privileges for Israel has ended. Abraham's true children will remain in his household (the new covenant) and enjoy its privileges permanently; but Abraham's slaves (think of Hagar) will be driven out. Only a son enjoys freedom. If therefore the Son of God will make them free, they will be free indeed.[6]

In view of the teaching of Christ on the true children of Abraham a little later, Hendriksen is probably correct in seeing Ishmael as the bondservant, and Isaac as the "son" of this verse. The distinction between the true sons of Abraham, that is, the "spiritual seed" and the mere fleshly descendants, is of utmost importance in understanding the Scriptures. Christ is the true "seed" of Abraham; and all of the "spiritual seed" of Abraham are "in Christ."