On sin:

He that hath slight thoughts of sin never had great thoughts of God.
— John Owen

No man ever fell into error through being too watchful.
— Charles Spurgeon

The best things which we do have somewhat in them to be pardoned.
– J. C. Ryle

In heaven the chief thing that we look for, next to the presence of Christ,
is complete separation from all sin.
– J. C. Ryle