Pietism
Though we all ought to wholeheartedly embrace and emphasize the importance of Christian piety, we must remember that piety is built on doctrine. Once doctrinal beliefs disappear, piety is emptied of the gospel and it becomes a form without content.. So we must istinguish between Piety and Pietism: The former is the result, consequence, outworking of one's union with the Lord Jesus Christ. The later, is usually used as a pretense of righteousness, erroneously thinking our work might earn merit before a holy God. When we recognize, however, that God demands holy perfection we behold our spiritual bankruptcy and so we look to Christ for mercy. Our work is then accepted in Christ but in no sense meritorous.
Pietism is a movement within Christianity that attempts to be inwardly focused on individual holiness and a consistent Christian life, sometimetimes at the expense of focusing on what Christ has done for us.
The danger of pietism is a lack of full, biblical assurance based upon the objective promises of God. (HC, Q&A 21 & LD 32)
By Scripture
Old Testament