Chapter XX had raised the issue of cheng, usually translated as 'sincerity' and meaning something like 'being true to one's human nature'. Chapter XXI takes this theme and begins the final line of development of the book, as we begin to learn the nature of the true sage, summarizing the essential principle that studious reflection or intelligence and sincerity interrelate. Some people, the sages, are such that being true to their nature issues without impediment into studious reflection; others there are whose studious reflection brings about their being true to their nature. Zhu Xi takes the next twelve chapters to comment directly on this.
Chapter XXII connects sincerity to the Confucian thesis of moral influence, and also the beginning of the claim that this moral influence is of cosmic scope. Sincerity is what allows one to express fully one's true nature. This allows one to influence others in the full expression of their true nature. In doing this, the wholly sincere can develop the true natures of animals and other things. In doing that, one can develop the elements and forces of Heaven and Earth; and in doing that, one becomes a triad with Heaven and Earth. The influence on other human beings obviously occurs by example and teaching. Zhu Xi suggests that the influence on animals and other things consists of coming to know them and giving them an appropriate role to play in one's own activities. In essence, we might say, the sincere person gives even the nonhuman world a moral destiny and value. Thus the Way of the Sage as it were unites the Way of Heaven and the Way of Earth.
But even those who have not yet attained this level of sincerity may be doing great things, by cultivating the sprouts of goodness in their nature in such a way that they may become sincere (XXIII). As this incipient sincerity develops, it begins expressing itself outwardly; in doing so, it becomes something that can be an example and lesson to others, moving them. The influence of the sage is that toward which this process tends.
Sincerity gives its possessor the ability to divine matters of good and bad, like a spiritual being (XXIV); sincerity is what it is to be complete as a self and therefore is the end to which the self is directed, but it also overflows into others by way of moral authority and knowledge (XXV). This sincerity is thus ceaselessly active which gives it endurance; endurance gives it an expansive self-expression capable of incorporating everything simply by manifesting itself, and it is this that makes the sage an equal of Heaven and Earth (XXVI). Without showiness, it shows forth; without moving, it moves; without trying, it succeeds (XXVI).
Chapter XXVII gives a famous praise of the Way of the Sage. Like water in the water-cycle, the influence of the sage goes out to all things on Earth and even rises to Heaven. All rules of ritual and propriety merely express facets of the sage's Dao, and it is the person of propriety and moral authority that walks it. Because of this, the noble who seek to attain this Way attempt to develop themselves through study and self-reflection according to the Mean discussed earlier in the work, and do so no matter the social positions they happen to occupy.
The Confucius Sinarum Philosophus, the seventeenth century Jesuit commentary on the Four Books, takes the description of the cosmic significance of the sage as a possible prophecy of Jesus Christ. This is the sort of thing one would not find in modern scholarship on Chinese philosophy, of course, but it is worth noting as (1) an early attempt to identify a connection between Chinese and Christian approaches, namely, by suggesting that what the Confucians speak about when they talk about the sage, Christians find in Christ; and (2) perceived links like this are what originally led to the spread of interest in Confucian philosophy in Europe, and the eventual result of Confucius, however understood or misunderstood, becoming a household name in the West.
The next chapters of the work comment on various features of Chapter XXVII. Chapters XXVIII and XXIX gives illustrations relevant to the sage and the noble being such in a way appropriate to their social positions. (There seems to be considerable confusion among commentators as to what is meant by "these three things" at the beginning of Chapter XXIX, because there isn't anything to which they obvious refer. Legge notes that some suggest that it means the ways of the three ancient kings; that Zhu Xi takes them to mean the prerogatives of kings mentioned in Chapter XXVIII; and that others, including himself, take them to mean virtue, station, and time.)
Chapters XXX through XXXII eulogize Confucius as an example of the sage. (Like Chapter II, it for an unknown reason refers to Confucius by a relatively uncommon name.) He brought forward the traditions of ancient times; he acted in harmony with Heaven above and in harmony with Earth below. His influence went everywhere, as one would expect from the description of the sage in Chapter XXVII. Chapter XXXI in particular gives a long list of attributes of a sage. Chapter XXXII returns us to the link between cheng or sincerity and the Way of the sage.
Zhu Xi, quite plausibly, takes the last chapter to serve as a compendious summary of the rest of the work. It consists of quotations from the Book of Odes that provides images which are then in commentary applied to the noble and the sage. The noble are not flashy or showy, preferring substance over style. The excellence of the noble lies in an activity no one can see -- careful self-examination. This forms their characters even when they seem to be doing nothing. But this does not imply that they have no influence; indeed, they can move others without having to resort to rewards and punishments, simply by example and advice. Through the virtues of the noble, others achieve peace and happiness. This is, in fact, the most complete form of moral authority possible, the one that makes the virtuous like Heaven itself: by an activity that is not seen or heard, their effects extend onward without limit.