The means of sanctification
In the practice of sanctification, are there disciplines we should be doing in the Christian life that will enrich our walk with God? What did God prescribe to us for our growth in godliness?
This question is a very practical one now, and it was a practical question back then during the Reformation. Medieval piety had plenty of options for those who were devout, ranging from fasting, pilgrimages, penance and joining monasteries or nunneries. Spiritual disciplines were promoted as actions or practices that one could engage in to grow in holiness. Far from being an arid desert, the medieval period was a great time for piety with a huge variety of options. Even for those seeking to join a monastry, one had many choices stretching from the Dominicans to the Benedictines to the lay Brethren of the Common Life. Whatever one wishes to say about the medieval period, a lack of spiritual exercises isn't it.
The Reformation came about in the midst of a great variety of spiritual exercises, and rejected almost all of them. Monasteries and nunneries were closed, mandatory fasting rules from meat on Fridays were purposefully violated, and all manner of "impiety" occurred. Martin Luther even had books of canon law burned together with the papal bull that excommunicated him from the Medieval Catholic Church. To promoters of Roman Catholic piety, the Reformation was a time of great impiety and gross wickedness.
The Reformers of course were not promoting impiety. But it is true that they were "impious" from the standpoint of medieval practice. That should inform us that the Reformation was a full rejection of the whole idea of "spiritual disciplines" or "exercises." Reformed piety is not about inventing set times of spiritual activities which one labor in before God. It is not about doing things before God, so what exactly is Reformed piety about?
The Reformed tradition uses the term "means of grace" (c.f. WSC 88) to speak about the gifts that God gives to believers for our benefit. In other words, instead of "spiritual disciplines" being about "ascending" to God with our piety, it is rather God that "descends" to us. For who will ascend to heaven to heaven to bring Christ down? (c.f. Rom. 10:6) Rather than us working our piety towards God, in the means of grace God descends to us and give us His grace. We do not engage in "works of piety" to become godly. Rather, we come to the means of grace and engage in them to receive grace from God. We do not come to give to God, but to receive from God.
The means of grace are the preaching of the Word, the partaking of the sacraments, and prayer (WSC 88, c.f. HC65). Notably absent from the list are fasting, frequency of set prayers, contemplative meditation, religious pilgrimages, or even "serving God" in "lay ministry" and parachurch organizations. Now, we know that fasting is appropriate (Mt. 6:16-18), but fasting is only appropriate as an extension of prayer, and done in the same spirit. In other words, fasting is not more spiritual if it is of a longer duration, or if it includes harsher conditions (e.g. fasting from water as well as from food). In fact, it is not about how spiritual one is at all when one fasts, or doesn't fast.
The means of grace, being God's gift, is for us to partake to get strength from God. It is for the weak, not for those looking to be more "spiritual." In fact, as being means of grace, it particularly unsuited for the "super-spiritual." For it is not the well who needs a doctor, but the sick (Mt. 9:12). In fact, if one thinks that he is really very spiritual, then one ought to forego the means of grace altogether! But of course to say that shows only that one is unaware of his sinfulness before God (1 Jn. 1:8,10)
Instead of "spiritual disciplines," Reformed piety and sanctification forego these as being worthless for true piety. Instead, we focus on the means of grace that God has given in, and partake of them as weak saints needing desperately the nourishment of Christ. Thus, when we fast, we may fast more because we are greatly burdened, but we do not pass judgment on another who doesn't. In fact, those who don't fast might be doing better spiritually than us at that time, and in that we rejoice for them.