1 Corinthians 5:1

Actually there is reported among you fornication, and such fornication as exists not even among the gentiles, that one of you should have his father's wife!

James 1:6-8

but let him ask in faith, nothing doubting; for he who doubts is like a wave of the sea, wind-driven and tossed.

2 Peter 2:14

having eyes full of lustfulness, and that cannot cease from sin; enticing unstable souls; having a heart exercised in covetousness; children of a curse;

2 Peter 3:16

as also in all his letters, speaking in them of these things; in which are some things hard to be understood, which the ignorant and unstable wrest, as they do also the other Scriptures, to their own destruction.

Treasury of Scripture Knowledge

Summary

Unstable as water, thou shalt not excel; because thou wentest up to thy father's bed; then defiledst thou it: he went up to my couch.

Bible References

Unstable

James 1:6
but let him ask in faith, nothing doubting; for he who doubts is like a wave of the sea, wind-driven and tossed.
2 Peter 2:14
having eyes full of lustfulness, and that cannot cease from sin; enticing unstable souls; having a heart exercised in covetousness; children of a curse;
2 Peter 3:16
as also in all his letters, speaking in them of these things; in which are some things hard to be understood, which the ignorant and unstable wrest, as they do also the other Scriptures, to their own destruction.

Because

1 Corinthians 5:1
Actually there is reported among you fornication, and such fornication as exists not even among the gentiles, that one of you should have his father's wife!