Psalm 51:10

"Create in me a pure heart, O God, and renew a steadfast spirit within me."

stead·fast

[sted-fast, -fahst, -fuhst]

adjective
1.
fixed in direction; steadily directed: a steadfast gaze.
2.
firm in purpose, resolution, faith, attachment, etc., as aperson: a steadfast friend.
3.
unwavering, as resolution, faith, adherence, etc.
4.
firmly established, as an institution or a state of affairs.
5.
firmly fixed in place or position.

pure

[pyoor]
adjective, pur·er, pur·est.
1.
free from anything of a different, inferior, or contaminatingkind; free from extraneous matter: pure gold; pure water.
2.
unmodified by an admixture; simple or homogeneous.
3.
of unmixed descent or ancestry: a pure breed of dog.
4.
free from foreign or inappropriate elements: pure Attic Greek.
5.
clear; free from blemishes: pure skin.