Two things about Patanjali are usually stated with more confidence than the evidence permits.
The first is his identity. A grammarian called Patanjali wrote a major commentary on Panini, probably in the second century BCE, and Indian tradition has often held the two to be one man, sometimes adding a medical treatise to make him the author of works purifying speech, mind and body. The philosophical text is normally dated considerably later, somewhere between the second and fourth centuries CE, on the basis of what it engages with, which makes the identification difficult. There is also a live question about whether the Sutras are the work of a single author at all, since the third and fourth chapters sit somewhat oddly against the first two, and some scholars treat the text as a compilation from existing material by an editor who supplied its frame.
The second is its standing. It is natural to assume that a text this central to the modern idea of yoga was always central in India, and recent scholarship has argued that it was not. It attracted commentaries and then, for a long stretch of the medieval and early modern period, comparatively little attention, with other yoga literature more widely used. Its present authority owes a great deal to the nineteenth and twentieth century revival, and particularly to Vivekananda, whose Raja Yoga of 1896 presented it to a Western audience as the definitive statement of yoga philosophy.