The slave does not remain in the house forever; the son remains forever.
35. Now the slave remaineth not in the house always. He adds a comparison, taken from the laws and from political law, to the effect that a slave, though he may have power for a time, yet is not the heir of the house; from which he infers that there is no perfect and durable freedom, but what is obtained through the Son In this...
We have in these verses, I. A comfortable doctrine laid down concerning the spiritual liberty of Christ's disciples, intended for the encouragement of those Jews that believed. Christ, knowing that his doctrine began to work upon some of his hearers, and perceiving that virtue had gone out of him, turned his discourse from the proud Pharisees, and addressed himself to those weak believers.
Commenting on John 8:31-37
I speak that which I have seen with my Father,.... This is an aggravation of the sin of the Jews, in seeking to kill Christ, on account of his doctrine, since it was not his own, but his Father's; was not merely human, but divine; was what he the only begotten Son, that lay in the bosom of his Father, had seen in his heart...