Think of and seriously consider that small sins are the mother of great sins, and the grandmother of great punishments.
As little sticks set the great ones on fire, and a wisp of straw often kindles a great block of wood, so we are drawn on by the lesser evils to greater, and by the just judgment of God suffered to fall into them, because we made no conscience of lesser. The lesser commandments are a rail about the greater, and no man grows downright wicked at first, but rises to it by degrees.
So for punishments. Nahab and Abihu for strange fire; Ananias and Sapphira keeping back part, Uzzah for touching the ark; the Bethshemites for looking into the ark. We may make little reckoning of sin, but God does not make little reckoning of sin; or else why has He given us these instances? So that this advantage in the spiritual life we have by this instance, that no sin should be accounted small that is committed against God who is great.
Adapted from The Complete Works of Thomas Manton, 15:375.