The necessity for a holy, pure life

Here are some words from one of my heroes, Robert Murray McCheyne, to a young minister in Christian ministry:

Do not forget the culture of the inner man - I mean of the heart. How
diligently the cavalry officer keeps his sabre clean and sharp; every stain he
rubs off with the greatest care. Remember you are God's sword, His instrument -
I trust a chosen vessel unto Him to bear His name. In great measure, according
to the purity and perfections of the instrument, will be the success. It is not
great talents God blesses so much as great likeness to Jesus. A holy minister is
an awful weapon in the hand of God.

From: Andrew A. Bonar, Memoirs of McCheyne (Chicago: Moody, 1978), 95.