He believed in what God had testified of regarding the state of the unrepentant. That hell is eternal. Allow me to quote him:
"There are only two kinds of people in the end: those who say to God, "Thy will be done," and those to whom God says, in the end, "Thy will be done." All that are in Hell, choose it. Without that self-choice there could be no Hell. No soul that seriously and constantly desires joy will ever miss it. Those who seek find. Those who knock it is opened.
― C.S. Lewis, The Great Divorce
and...
Jonathan L. Kvanvig writes:
"[C.S.] Lewis believes that the doors of hell are locked from the inside rather than from the outside. Thus, according to Lewis, if escape from hell never happens, it is not because God is not willing that it should happen. Instead, residence in hell is eternal because that is just what persons in hell have chosen for themselves.
Similarly, Dave Hunt (1996) writes:
We may rest assured that no one will suffer in hell who could by any means have been won to Christ in this life. God leaves no stone unturned to rescue all who would respond to the convicting and wooing of the Holy Spirit."