Many people, when trying to make a point, will quote some ancient philosopher or other prominent thinker. The reasoning is that this wise person should have known what he or she was talking about and so the quote is relevant to the conversation. Is this really the case, though?
There is a form of logical argument in which one cites an expert in order to support one's point. This is called an Appeal to Authority. For example, if you are arguing about what foods you should or should not eat you may quote your physician on the matter, "My doctor says I'm supposed to eat baked fish rather than deep fried." Since your physician is an authority on health, and your health in particular, this form of argument is an Appeal to Authority and is correct.
However, there is a related logical fallacy called the Appeal to Unqualified Authority. In this case, one tries to use the Appeal to Authority but quotes a person who is not really qualified to be an authority on the subject matter. For example, if you are arguing with your mechanic over repairs to your car and you say, "My doctor says the problem should have the fuel injectors," you have probably committed the Appeal to Unqualified Authority fallacy. It is unlikely, but not impossible, that your physician knows more about auto mechanics than the professional mechanic.
You can see how there is a subtle difference between the good argument and the fallacious one. Even though the physician is an authority, that knowledge is in a limited field. It gives neither authority, knowledge, insight, nor wisdom in the rest of the world.
How would this apply to the great thinkers of the world? That's simple.
Almost every civilization has produced philosophers, thinkers, and scientists of some type. Those people are limited to their understanding of the world as filtered through the conditions and beliefs of that time and place. Though there may be some consistencies in humanity that withstand the changes in time, there are other aspects that do not translate easily. This must be considered when determining whether the philosopher's work is valid in a modern argument.
As an example, take Karl Marx. This fellow was educated and read up on the various philosophers of the Enlightenment. Many people have used his Communist Manifesto (among others) for various political and social arguments. Is that valid?
Marx lived in Europe while the Industrial Revolution was still raging on. Old social structures were in upheaval. Much of society wasn't sure what to do with itself. Science and technology were presenting a serious threat to the theocratic establishment for the first time (to most people's views.) Marx predicted that people themselves would gradually change society to remove capitalism and eventually a Utopia would sort of spread out across the world.
Many things have changed since then. The Industrial Revolution is long since over and we have entered the computer age (after a short jaunt through the space age.) The majority of people, even religious people, accept Evolution and the heliocentric solar system as fact. Many diseases have been wiped out. Marx's spontaneous revolution never happened.
Does this mean that Marx was an idiot? No, he just lived in different times and had a different perspective. You can still quote him if you are trying to support a point, but you probably need to be ready to defend the relevance of his words as they apply to your modern point. What he said may no longer be valid. On the other hand, maybe it will be.
All of this applies to anyone else you quote. Socrates has been dead for thousands of years. Bono of the rock band "U2" is a rock star. Each has a field of endeavor where he is an authority. You have to ask yourself, is he an authority in the area in which I'm going to quote him?