One dead fly

(Thomas Brooks, "A Cabinet of Choice Jewels" 1669)

No hypocrite is totally divorced from the love and liking 
of every known sin. There is still some secret lust, which 
as a sweet morsel he rolls under his tongue, and will not 
spit it out. Every hypocrite lives under the dominion and 
reign of one base lust or another—and will do what he 
can to save the life of his sin—though it be with the loss 
of his soul. A hypocrite always reserves one nest-egg or 
another in his heart or life, for Satan to sit and brood on. 

O sirs! Satan can hold a man fast enough by one sin, 
as the fowler can hold the bird fast enough by one claw. 
Satan knows, that one sin lived in and allowed, will as 
certainly damn a man as many sins; just as one disease, 
one ulcerous part, may as certainly kill a man as many. 
One dead fly will mar the whole box of precious ointment.
One jarring string will bring the sweetest music out of tune.

If the leper in the law had the spot of leprosy in any one 
part of his body, he was accounted a leper; although all 
the rest of his body was sound and whole, Lev. 14. Just 
so, he who has the spot of the leprosy of sin allowed in 
any one part of his soul, he is a spiritual leper in the eye 
of God; he is unclean, though in other parts he may not 
be unclean. 

If a swine does but wallow in one miry or dirty hole—it 
is filthy; and certainly, that soul which does but wallow
in any one sin—he is filthy in the eye of God. 

O sirs! remember that . . .
  as one hole in a ship will sink it, and
  as one stab at the heart will kill a man, and
  as one glass of poison will poison a man, and
  as one act of treason will make a man a traitor,
so one sin lived in and allowed, will damn a man forever! 

One millstone will sink a man to the bottom of the sea as 
well as a hundred. Just so, one sin lived in and indulged,
will sink a man to the bottom of hell as well as a hundred.