A.T. Robertson Commentary Galatians 5:14

"For the whole law is fulfilled in one word, [even] in this: Thou shalt love thy neighbor as thyself." — Galatians 5:14 (ASV)

Even in this (εν τω). Just the article with εν, "in the," but it points at the quotation from Le 19:18. Jews (Luke 10:29) confined "neighbour" (πλησιον) to Jews. Paul uses here a striking paradox by urging obedience to the law against which he has been arguing, but this is the moral law as proof of the new love and life. See also Ro 13:8, precisely as Jesus did (Matthew 22:40).