Verse 43 has two parts: love thy neighbor, and hate thine enemy. The first of the parts has a direct correspondence in the Septuaging version of Leviticus 19:18 (Robert Guelich. A Foundation for Understanding the Sermon on the Mount. Word Publishing, Dallas. 1982, p. 225). The second part does not have a scriptural referent, and it was probably the presumed corollary to loving a neighbor, where “love” inverts to “hate” and “neighbor” (or fellow Israelite) inverts to “enemy.” The social principle upon which this statement is founded is the understanding of insider/outsider boundaries. One’s “neighbor” is someone included in some social boundary. One’s “enemy” is virtually by definition outside the boundary.
“The first-century use of neighbor had narrowed in its reference from that of Leviticus 19:18. Whereas neighbor meant fellow member of the covenant community, an “Israelite,” in the Old Testament context in contrast to the non-Israelite, the rabbinic discussions contain many examples of the struggles with the casuistic implications of neighbor. Qumran had clearly limited the designation to members of their community …” (Robert Guelich. A Foundation for Understanding the Sermon on the Mount. Word Publishing, Dallas. 1982, p. 225).
The reversal in this pairing is to extend insider courtesy to those typically denied such courtesy, the outsiders, or the enemies. It would be quite normal to assume that one would love a neighbor, a fellow insider. One might bless them. One might do good to them. One might pray for them. Jesus requires that we expand ourselves outside of our circle. We are not to make such insider/outsider distinctions, and even those who might be considered enemies should be included in our social interactions as though they were insiders themselves.
Old World Context: The social conditions of first-century Israel provide poignant backdrop for the need to heed the nature of one’s interactions with outsiders. The presence of the powerful Romans created tension, and the tensions would eventually escalate into rebellion and a brutal Roman retaliation. Into this heated climate Jesus contends that the us/them distinction should be removed. We are to see ourselves as part of the entire world, not simply as a persecuted subset. There is an interesting shift from verse 42 to 43 where the singular enemy of the setup phrase becomes the plural enemies of the transformed gospel requirement. This suggests that the focus was on the individual relationships rather than national enemies (Robert Guelich. A Foundation for Understanding the Sermon on the Mount. Word Publishing, Dallas. 1982, p. 228).
Book of Mormon Context: The most obvious insider/outsider distinction in the Book of Mormon was the Nephite/Lamanite distinction. In the New World, this command to love one’s enemies would have been perceived as a command to include the Lamanites as insiders.