In Galatians 5:14, Paul alludes to Leviticus 19:18: the Law is fulfilled in one commandment, “Love your neighbor as yourself.” This verse is the most quoted verse from the Pentateuch in the New Testament, despite the fact that it is almost never referred to in contemporary Jewish texts. Perhaps this is because Jesus himself stressed love of neighbor as a fulfillment of the law!
But what does Paul mean when he says that the whole Law is fulfilled in a single commandment?
The verb “fulfill” (πληρόω) in verse 14 is a perfect passive, indicating that the completion of the Law has already been accomplished when believer “loves his neighbor as yourself.” Paul’s point is that if one is loving one’s neighbor as themselves, then they are already doing the “spirit of the Law.” By walking in the Spirit the believer is already fulfilling the whole law, but the requirements of the Law were never required for the Gentile believer in Christ (Witherington, Galatians, 381).
Defining who is one’s neighbor was also a point of discussion, especially in the Good Samaritan parable. Jesus is asked by an expert in the Law to define “neighbor.” The lawyer likely understood the word to refer to fellow Jews, since that is what it means in Leviticus. Jesus expands this to include anyone who is in need. It is possible Paul has fellow-Christians in mind in Galatians 5:14, since the context is factions within the church (5:15, 26). A few verses later Paul expands on what he means by doing of good, in 6:10 he says that the believer in Christ ought to do good for everyone, but especially the “household of faith.”
Paul’s point is not, “if you want to keep the law, love your neighbor.” He has said repeatedly that the age of the Law is done and over with and the one who is in Christ is free from the Mosaic Law. Likewise, Paul would not agree that the believer must do good in order to “stay right with God.” The problem for Paul is “Why should a believer do good if it does not make them saved nor keep them saved?”