'Grace', a mistranslated word and misunderstood concept
The word charis, often translated 'grace' is quite heavily used in the New Testament. For many Christians and denominations, particular understandings of 'grace' shape their understanding of Christianity. Even though it means different things to different Christians, 'grace' (charis) is commonly heavily used and an important theological and emotional term. It is therefore, in my view, immensely important to accurately understand the meaning of the Greek word charis as it was understood by the original writers and readers of the New Testament. A multitude of sins, eisegesis, and bad theology, can be built on a misunderstanding of this word.
Amongst Classics scholars there is no debate as to what this word means. Surviving documents from the ancient world contain hundreds of passages that give us great clarity about their understanding of charis and the role this word and its concepts played in their society. Charis was the key-word in what scholars call "the reciprocity system". This system operated according to time-delay exchanges where goods were given and then at a later time goods of relatively equal value were returned to the giver. These goods could be tangible (money, material goods) or intangible (public acclaim, authority). An obligation existed to repay favors owed, they were not 'free' in the sense we would understand it - it is just like when a bank gives you a loan the money is not 'free'. (It is due to this reciprocal nature of the transactions that scholars label it the Reciprocity System.) Essentially the system was an informal system of economics. The word charis itself is best translated with the English "favor" in the sense of talking about favors given and favors repaid. Greek makes no linguistic distinction between the first favor given and the second favor to repay it, calling each a charis. Greek also uses charis to refer to a positive attitude toward someone - we would speak in English of this as "regarding them favorably" or "having their favor".
Of course, a century or more ago, such information simply wasn't available. People interpreting charis in the Bible had to use what information they had and try to make some sense of it. Reformation Christianity is famous for seeing charis as being "free grace" and being the opposite of human effort. These concepts have heavily influenced many Christians' understandings of 'grace' today, but have nothing to do with the actual meaning of charis in Greek. The translation 'grace' is not a good one, it is not 'free', and it isn't the opposite of human effort.
These historical misinterpretations of 'grace' have led to correspondingly incorrect interpretations of passages that use charis. Romans 4, for example, contrasts the Reciprocity System to a Contractual system (a rather subtle contrast) which has historically been exegeted as the difference between human effort and reliance on 'grace'. Similarly Ephesians 2:8, due to the ambiguity in Greek about givers and receivers of favors clarifies that God is the giver of the favor and we the receiver, and yet this has historically been exegeted as speaking about lack of human effort.
Unfortunately, nothing endures and propagates quite like bad theology. At certain points in history, theologians have constructed theologies based on certain incorrect understandings of 'grace' and these theologies remain influential today and taught as biblical even when scholarship regarding the meaning of these words has long moved on. Mistaken ideas about charis continue to influence many Christians who are convinced that 'grace' means salvation is in no way by human effort.