The church has struggled with this for 2000 years: which is more important, doctrine or piety? The struggle comes from our innate human tendency to see things as either/or when it should be both/and. I think this is engraved in our DNA. Everyone seems to struggle with this issue. Some certainly struggle less or overcome it easier but I believe that everyone has this. Part of our schooling is to learn the critical thinking skills necessary to recognize when things are either/or and when they must be both/and. The excluded middle certainly is not always in view and we can often commit the fallacy of trying to steer a middle course where none exists. However, it seems just as common as failing to recognize the need to hold two seemingly opposite ideas in tension.
In my religious background of 20th Century American Fundamentalism there has been a significant struggle with the issue of doctrine vs. piety. Anyone who has spent time in fundamentalist circles has heard the old saws: "Churches are dying by [seminary] degrees!" "We don't need to read all that theology--we have the Bible!" "Calvinism [or some other doctrine-you-love-to-hate] kills evangelical fervor!" Of course no one can be completely without doctrine since doctrine is simply "a set of beliefs held and taught" (Oxford American Dictionary). So even the belief that "we don't need doctrine" would be a doctrine! Doctrine is perhaps best thought of as how we think about our religion.
Piety is a recurrent theme in church history as well it should be. Piety is "the quality of being religious or reverent" and often carries with it the ideas of "devotion, piousness, religion, holiness, godliness,saintliness; veneration, reverence, faith, religious duty, spirituality, religious zeal, or fervor." In other words it reflects in large part how we feel about religion or how religion affects our emotions.
As we look through church history we often find movements or periods or denominations in which a dry doctrinaire attitude has pervaded and characterized the people and assemblies. This is never a good thing. For to paraphrase Paul's letter to the Corinthians: if we understand all the mysteries of God and we have no sentiment it is worthless. Yet the solution cannot be boundless piety.
Pietism has often arisen as a reaction to dead orthodoxy and as such it is very good and even necessary. However, those carried away in spirituality often fail to realize that piety can lead to as many problems as lifeless theology. By rejecting as unnecessary the intellectual pursuit of doctrine as derived from the revelation of God, the zealous often fall into the same kinds of error that the church has been rejecting for 2 millennia. Think about this: most christian cults, from the gnostics to the Branch Davidians, start out at least cloaked in pietism. Mormonism was about purifying a corrupt Christianity. The Watchtower society is renowned for their keeping of the moral law. No one is stricter about allowing God to affect their lives than the 7th Day Adventist. But all of these groups have strayed away from orthodoxy. And what of those who would not be considered cults. Within the accepted bounds of orthodoxy those who reject as superfluous the scientific study of scripture and history lay out a course that quickly steers for the rocks of heterodoxy. This is most evident today in many fundamentalists visceral concern about the "evils" of Calvinism. While I am no calvinist, I do know that calvinism is within the bounds of orthodoxy and always has been. But, those who set out to be not just arminian but outright anti-calvinist quickly stray into open theism. This is natural since the only coherent position to take against the reformed emphasis on the sovereignty of God is to posit a "limited sovereignty" that is Open Theism. While no self-avowed fundamentalist would take the label of open theism there are certainly many who sound like Clark Pinnock and his friends.
So, religion is a lot like sailing. You really cannot sail all on one tack. If you want to go anywhere you have to tack back and forth. If you want to sail a fairly straight course close to the wind you often have to tack quite frequently. Some people want to pretend that religion is like a powerboat. The power is doctrine or piety and they want to firewall the engine and point the bow in a direction and go. The reality is that we need correct doctrine. Doctrine that is grounded in the Word of God. Necessarily any doctrine that distinctly departs from the way the church has been reading the Bible for 2000 years is no good teaching. We might arrive at a fuller understanding of the end times than the medieval church had but we cannot derive some kind of idea that Christ is not ever returning since the church has always affirmed the imminent return of Jesus. We also need piety. The biblical teach of the gospel, the good news that God is seeking to correct our dangerous predicament of rebellion against Him by adopting us through the aegis of Christ's perfect life and vicarious death, is something that must move our hearts. God's revelation is not just given to us to address our brains but also to rend our hearts and shake our knees. This is most clearly seen in the Apocalypse of St John when the Lamb is revealed before everyone who has ever lived on the earth. The response is twofold: they bow in perfect pious worship and they verbally affirm the biblical truth that God and the Lamb are who they have revealed themselves to be!
We need to live our lives in expectation of heaven where we will achieve the perfect balance of piety and doctrine.