These articles examine the Bible's concept of love, and the foundational issues that help us to understand the focus of the passage.
"Love" in the Old and New Testaments; the N.T. concept of "Love," as used in this passage (v. 9a)
"Love must be sincere."
A comparison of the Old and New Testaments (the concept of "love" is common to both testaments, in spite of what some people would have you believe). Also a quick look at 1 Corinthians 13, commonly called "the Love Chapter." (Though not our emphasis, 1 Corinthians 13 is a good foundation on which to begin our study.)
There are four different Greek words that were used at the time the New Testament was written. Here, we discover which concept of "love" is the focus of this passage. We also find out why we describe this section of Romans 12 as being a description of "Sincere, Unhypocritical Love."
Issues related to "evil vs. good" (verse 9b and 21)
"Hate what is evil; cling to what is good. ... Do not be overcome by evil, but overcome evil with good."
God saves people for a purpose: for following Jesus. There are necessary changes that must occur, which involve both our attitude and our actions. These changes are summarized in the concept of hating evil and clinging to good (v.9b), so that the good overcomes the evil (v. 21). All the verses in-between (vs. 10-20) illustrate how this is to be done.
Hate is not always the opposite of love. Sometimes it is a necessary part of love... and without it, the "love" is fake.