Christian piety is a good thing. Webster, in his 1828 dictionary defines piety as "in principle, is a compound of veneration or reverence of the Supreme Being and love of his character, or veneration accompanied with love; and piety in practice, is the exercise of these affections in obedience to his will and devotion to his service. Piety is the only proper and adequate relief of decaying man."
Thus it is that Paul would admonish the children and grandchildren of a widow in the practical application of piety, "But if any widow has children or grandchildren, let them first learn to show piety at home and to repay their parents; for this is good and acceptable before God." 1 Timothy 5:4 NKJV
Pietism, on the other hand, is an evil cancer that easily and unnoticeably creeps in to destroy the unity of Christ's Church. It is a false aberration of piety. Piety is faithfully discharging our duty to God according to His Law and commandments toward ourselves, family, church, and state. Pietism is imputing upon others that which God requires of me with disregard to what God might or might not require of another.
Jesus often rebuked the scribes and Pharisees for their pietism. "Woe to you, scribes and Pharisees, hypocrites! For you pay tithe of mint and anise and cummin, and have neglected the weightier matters of the law: justice and mercy and faith. These you ought to have done, without leaving the others undone. Blind guides, who strain out a gnat and swallow a camel!" (Matt 23:23,24) With regards to pietism, Rushdoony put it this way,
The influence of pietism has been an important one in modern history, and it has borne false witness concerning God's requirements. Its emphasis on sinless perfection has in fact begotten sin. Where men expect a sinless perfection of other men, they are readily led into a sinful intolerance of human frailties. This sinful perfectionism especially abounds at the end of an age, or in any era when men find their problems either temporally or permanently unsurmountable [sic]. Where problems are insoluble, men then turn on one another. Their basic unhappiness over the insoluble problems manifests itself in trying to dissolve from their midst people who irk them. As the fall of Rome began to loom, men long before had fled from the cities, recognizing their hopeless future. Their reaction, however, was far from sound. Christians and pagans alike turned on men and renounced them, becoming desert hermits. But being alone solved nothing, and the inner torments of these desert refugees indicated that their flight gave them neither peace nor an answer to the worlds' problems. Today again, as problems appear to be insoluble, the irritation of man with man increases. There is a low level of tolerance with children, neighbors, husbands, wives, friends, and associates. Instead of solving problems, this kind of perfectionism only aggravates them. Giving an exaggerated emphasis to human frailties is to bear false witness concerning them. The law here is plain-speaking: Bear ye one another's burdens, and so fulfil the law of Christ (Gal 6:5) This has reference, very clearly, as Galatians 6:3-5 makes clear, to our faults and weaknesses We are to recognize that each of us has a burden of weaknesses, and every man shall bear his own burden (Gal 6:5) We sometimes need correcting, but, much of the time, we must live together mindful of our common frailties and work together to gain that maturity which comes from seeking first the Kingdom of God and His righteousness (Matt 7:33). – Rushdoony; The Institutes of Biblical Law; page 630
Pietism is an evil, destructive, and killing cancer. It is a Trojan Horse – it looks good on the outside but that which lies within is deadly. Personal piety is to be desired and developed. Pietism, on the other hand, always, always, with no exception, breeds strife and hypocrisy. Flee from pietism.