At the very least, people should read Tsongkhapa in his own words, and in the words of his two direct disciples, Gyaltsabje and Khedrubje. Later commentaries are suitable introductions and supplements, but a Gelugpa not reading Tsongkhapa is like a Vaishnava not reading the Bhagavad Gita or Shia and Sunni Muslims not reading their respective hadiths. Knowledge is power, and everyone who wishes to follow Tsongkhapa's tradition deserves nothing less than reading him in his own words and emulating the approach he advises in his works.
This statement from Zilkar Rinpoche sums up what I believe to be the truest way of honouring Lama Tzongkhapa through our study and practice: Following LTK's advice, and with courage, approaching the Indian treatises ourselves to form our own opinions, rather than taking someone else's word for it:
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If one wants to study “the Great Treatise on the Stages of the Path to Enlightenment”(Lam Rim Chenmo) in depth, one needs to supplement one’s practice with the study of the five major treatises examining the subjects written in the Great Treatise. Why is this necessary?
The first reason is that, while “The Great Treatise on the Stages of the Path to Enlightenment” authored by Je Tsongkapa is a set of conclusions based on his own experiences and realizations, he did not really elaborate on the reasons and the details that support his conclusions. Thus, we will need to learn the rationale and logic behind the conclusions during the course of our study. If we focus only on the conclusions found in the text, the conclusions will always be Je Tsongkapa’s conclusions, and not our conclusions. Therefore, we must learn what it is that supports these conclusions.
Secondly, what are covered in the great treatise are essential, concise teachings, comments, and instructions. Many philosophical theories were not explained explicitly and completely, and thus need to be supplemented by the study of the five treaties.
For example, the section of “Taking Refuge to the Three Jewels” dealing with persons of lower capacity of the Great Treatise must be supplemented with the explanations in the “Uttaratantra” and in “The Ornament for the Mahayana Sutras”. The theories of karma and its effect in the Great Treatise are sourced mainly from the Abhidharmakosha-shastra, so if we only study the Great Treatise, we may not have a solid understanding of the subject. In addition, the subject of karma and its effect in the Abhidharmakosha-shastra is presented from a particular Tenet’s point of view that incorporates the capacity of the learner to comprehend it. If we do not understand this, we will not be able to comprehend the explanation of karma and its effect in the Abhidharmakosha-shastra as well.
Furthermore, in the section of the Great Treatise dealing with persons of medium capacity where the Four Noble truths are taught, the subject such as the sixteen aspects of the Four Noble Truths frequently quoted in various scriptures are not explained in detail. In addition, the Truth of the Cessation and the Truth of the Path were not explained. Therefore, supplemental study has to be undertaken.
Again, in “The Ornament of Clear Realization,” the section dealing with the Bodhicitta for persons of great capacity, twenty-one different kinds of Bodhicitta and the ways of cultivating them are mentioned, but none of these points are covered in the Great Treatise. The Calm Abiding (Samatha) section should also be supplemented with the “Sravaka Level” and the “Bodhisattva Level” contained in the “Compendium of Determinations”. We should refer to the viewpoints found in the Abhidharmakosha-shastra, the “Sravaka Level,” the “Bodhisattva Level,” and the “Compendium of Knowledge”, when studying the mental afflictions, which is the Truth of Cause in the Four Noble Truths.
In summary, the essence of Buddha’s teaching is embodied in the three volumes of the Perfection Wisdom Sutra. Because the Perfection Wisdom Sutra is immensely vast, deep, and difficult to fathom, we need to study “Four Tenets “, “Entering to the Middle Way”, “Fundamental Verses on the Middle Way”, and the “Special Insight” section of the “Great Treatise”, in order to understand the profound view of emptiness directly and explicitly presented in the Perfection Wisdom Sutra. In order for us to understand the extensive deeds or the method side of the teaching, which is indirectly and implicitly presented in the Perfection Wisdom Sutra, we need to study the “Grounds and Paths”, “The Eight Categories and Their Seventy Topics” and “The Ornament of Clear Realization”.