Loving one another evidences two realities in our lives according to verse 12: God abides in us and his love is brought to completion in us. John is very fond of using this word “abide.” It is common in his Gospel as well as in this letter. In verses 12–16 alone, John uses this word six times. To abide in God (or Christ) not only is a statement of fact concerning our relationship to God and his relationship to us, but it is also a statement concerning our conduct. When we abide in Christ, we are behaving according to his character. This is why John links the concept of abiding with that of loving in these verses.
In verse 13, John makes the important point that we have assurance we are truly saved because God has given us his Holy Spirit to indwell our lives. The presence of the Holy Spirit in our lives is the means of assurance that we are truly united to Christ. This is what John means when he speaks of our “abiding” in God or Christ and and he in us.” If we “abide” in him, we are going to reflect his character in our lives. Our conduct towards others will be commensurate with God’s character.
Three things are inevitable consequences of dwelling in love. First, to dwell in love is proof of the fact that God dwells in us and that we are in God (v. 16). Second, this is the demonstration of the fact that love has been perfected in us (v. 17). This is the issue of our sanctification. Third, we have boldness before God in the day of judgment (v. 17). The day of judgment will be something formal, public, and final. Christ Himself will be the Judge (John 5:27, Matthew 25:31, Acts 10:42 and 17:31).
In a sense, this entire paragraph is a further amplification of the previous paragraph, 4:7-11. At the very least, this paragraph further amplifies the thought of the exhortation in v. 11 of the previous paragraph.
In terms of sermon structure, the paragraph has four parts that could be represented in the following outline:
I. We should love one another because:
A. God’s Love is Completed in Us When We Abide in Him – (v. 12).
B. The Spirit’s Testimony to Christ is Our Testimony – (vv. 13–15).
C. Love Brings Confidence in the Day of Judgment – (vv. 16–19).
D. It is Impossible to Love God and Hate Your Brother – (vv. 20–21).