As you know, Tasawwuf is a science, the science of self-purification. It is a branch of Islam just as valid as the other branches such as Tafsir (science of explaining the Quran) and Fiqh (science of extracting rulings).
According to Maulana Thanvi, there were two theories as to how self-purification should come about. What method should we use that will make us pious and righteous Muslims? One was to concentrate on letting go of the sins that we are accustomed to commiting, and once that is achieved, we should then work on improving and increasing our good deeds and worship. The other way was to start loads of zikr, worship and good deeds immediately and the sins would then slink away themselves. Though both methods are good and beneficial, the first method was more preferred by the Chishti's whilst the latter by the Naqshbandi's (this makes it clear why there are different Tariqa's, they differ in methodology but the goal is the same).
In the first instance, the priority is given to shunning his sins since they argue that this is the most important for a Muslim and supercedes nafl worship (as the Prophet [peace be upon him] said: If you guard against the forbidden things, you will be the most devout of men' Ahmad, Tirmidhi). Maulana Thanvi says that it is like when doctors want to give medicine to remedy the disease and then concentrate on strengthening the patient but the downside is that the person may while his life away in this battle and not gather many good deeds and dhikr in the time - he may even lose hope and give up.
The second group says that the person should immerse himself in zikr and ibadah, and this should itself help to erode the bad habits he has (Allah says 'establish regular Prayer: for Prayer restrains from shameful and unjust deeds [29:45]). The danger with this is that the person may become complacent, thinking that since he does so much worship, one sin here and there won't really matter. Maulana Thanvi compares it to feeding a person strengthening potions when he is ill - it'll probably just strengthen the persons illness, making it worse. (Maulana was a doctor specialising in herbal medicine).
The great Maulana then comes to his own conclusion, which I will detail insha Allah in the next post.