One of you is a devil!

(Thomas Watson, "The Christian Soldier" 1669)

"Examine yourselves to see whether you are in the faith;
 test yourselves. Do you not realize that Christ Jesus is in
 you—unless, of course, you fail the test?" 2 Corinthians 13:5

Self-examination is a necessary—but difficult work.

Self-examination is the setting up a court in conscience and
keeping a register there, that by strict scrutiny a man may
know how things stand between God and his own soul. By
a serious scrutiny of our hearts, we come to know to what
prince we belong—whether to the Prince of Peace, or the
prince of darkness.

Self-searching is a heart-anatomy. As a surgeon, when he
makes a dissection in the body, discovers the inward parts,
the heart, liver, and arteries—just so, a Christian anatomizes
himself.

Sentimentality and public opinion are false rules to go by.
We must judge the state of souls by the light of Scripture.

Many have foolish, presumptuous hopes. They fancy their
state to be good; and while they weigh themselves in the
balance of presumption, they pass the test.

Many take their salvation on trust. The foolish virgins thought
they had oil in their lamps, the same as the wise. How confident
are some of salvation—yet never examine their title to Heaven.

Many rest in the good opinions of others. How vain is this!
Alas, one may be gold and pearl in the eye of others—yet God
may judge him to be reprobate silver! Others may think him a
saint—and God may write him down in His black-book! Judas
was looked upon by the rest of the Apostles as a true believer
—yet he was a traitor! "Then Jesus replied—Have I not chosen
you, the Twelve? Yet one of you is a devil!" John 6:70

Others can but see the outward behavior—but they cannot
tell what evil is in the heart. Fair streams may run on the
top of a river—but vermin may lay at the bottom!