As the modern science progresses, we find more and more things which match those written in ancient texts. For example the orbits of various planets were accurately known 100s of years before they were published as they have been found so in the Vedas, the modern science even the higgs boson "god particle" seems to point towards divinty, is it really possible that we are just rediscovering long lost knowledge ?
Old ideas have a way of coming back to life, but not in they were once known; Kant remarked in his critique of pure reason that many ideas have a kind of family resemblence at a distance, but studied more closely their differences become more apparent.
For example: atomism in Greek antiquity & atomism today, or the Jain Anekantavada (doctrine of plurality) and post-modernisms relativising of grand-narratives; the One theorised in the Abramaic tradition, Brahman in the Indian Upanishads, and the tao in China; Jain ecological thinking and modern enviromentalism; Indras nets & fractals; Euclids axiomatics of geometry and Paninis of Sanskrit; feminism and female shakti;
What is a house, a cabin, a place but a cave that man constructs himself?
Many more examples can be found.
What this shows about the ecology of ideas, thoughts, traditions & beliefs is worth reflecting on; one might suppose that because ideas are transmitted and dispersed without their history that often the connection isn't seen between old and new; for example modern atomism is connected to greek antiquity via Newton who read Lucretious On Nature which gave an exposition of atomism (in verse).
Generally, these angles are pursued by historians of science, philosophy and religion. They tend keep to within a tradition - due to pressures of language, politics and religion.
For example the clear (to me at least) resemblence between Spinozas God and the Islamic God, in several perspectives, is not remarked on generally; especially when one considers that Spinoza, though Jewish, was a Portugeuse Jew whose family left Al-Andalus (the Iberian peninsula under the Ummayad Caliphate) after the Reconquista turned into the Inquisition.
That is the division of language, religion and politics sever the natural history of an idea and help it change and adapt.
Whereas during the classical revival during the Renaissance the gaze was directed backwards, in saluting the past; in the modern era, the gaze is directed towards the future, the past is negated, history forgotten; One rather thinks of here the artistic movement born in Italy Futurism which celebrated everything new - youth given primacy, technology glorified; speed worshipped and violence itself become beautiful.
No, we don't know all we need to know in any reasonable sense.
People die all the time of cancer. If we knew more, they wouldn't. (They die less now than they did fifty years ago because we've learned a lot in the meantime.)
Even philosophically, we've had great insights into the nature of perception and cognition in the past hundred years or so courtesy of neuroscience, and there is good reason to believe that more will be forthcoming on topics as diverse as ethics, qualia, and the nature of knowledge.
And we don't know how to fly off our planet before our sun renders it uninhabitable. That might be kind of important someday. (Heck, not long ago we didn't even realize that this would someday be a problem.)
First of of all how do you define all we need to know?
For sure presently we do not know all that is possible to know. Not all theorems have been proven, not all species of animals, plants bacteria etc. have been discovered, not all stars in our galaxy and galaxies in our universe have been discovered, not all scientific and philosophical theories have been formulated and I could add really so much more to this list.
Despite our universe can be finite and may have only a finite number of things to be discovered the capacity of generation of our mind is unlimited in principle (limited mostly by our capacity to process information, store information and our life span in terms of individual and the whole mankind and eventually bounded by the limits of the universe itself if any).
During the enlightenment they formulated the concept of progress; the ideas of ancient time in Europe (Greek philosophy) were always kept mind (and never rediscovered) and used as inspiration to formulate modern theories. For instance the atomic theory of Democritus was used in the 1800's to formulate the modern atomic theory. I read the "Tao of physics " by Capra, and I can say that definitely Asian ancient philosophies had some qualitative insight in physics. They were rediscovered only in western countries because in Asia scholars never forgot them. Anyway the relationship between those theories and modern physics is comparable to the relationships between the atomic theory of Democritus and modern particle theories.