Love is intrinsic to God’s character. It is also a critical arbiter for distinguishing who is—and who isn’t—one of God’s people.
In writing his first epistle, the apostle John’s goal was to help struggling believers gain assurance of their salvation. He states his purpose in 1 John 5:13: “These things I have written to you who believe in the name of the Son of God, so that you may know that you have eternal life” (emphasis added).
But 1 John also serves a secondary purpose, and that is to destroy the false assurance of those who may profess faith in Christ without really knowing Him. Therefore, he writes such things as, “If we say that we have fellowship with Him and yet walk in the darkness, we lie and do not practice the truth” (1 John 1:6). And, “The one who says, ‘I have come to know Him,’ and does not keep His commandments, is a liar, and the truth is not in him” (1 John 2:4). And, “The one who says he is in the Light and yet hates his brother is in the darkness until now” (1 John 2:9).
Here he makes godly love a kind of litmus test for the true Christian: “The one who does not love does not know God, for God is love” (1 John 4:8). With regard to that statement, Martyn Lloyd-Jones observed,
John does not put this merely as an exhortation. He puts it in such a way that it becomes a desperately serious matter, and I almost tremble as I proclaim this doctrine. There are people who are unloving, unkind, always criticizing, whispering, backbiting, pleased when they hear something against another Christian. Oh, my heart grieves and bleeds for them as I think of them; they are pronouncing and proclaiming that they are not born of God. They are outside the life of God; and I repeat, there is no hope for such people unless they repent and turn to Him.  D. Martyn Lloyd-Jones, The Love of God (Wheaton, IL: Crossway, 1994), 45.
Sadly, most of us have encountered professing Christians whose hearts seem bereft of any genuine love. The apostle John’s admonition is a solemn reminder that a mere pretense of faith in Christ is worthless. Genuine faith will inevitably be shown by love. After all, real faith works through love (Galatians 5:6).
This sort of God-given love is not easily counterfeited. Look at all that is involved: love for God Himself (1 Corinthians 16:22); love for the brethren (1 John 3:14); love of truth and righteousness (Romans 6:17–18); love for the Word of God (Psalm 1:2); and even love for one’s enemies! (Matthew 5:44). Such love is contrary to human nature. It is antithetical to our natural selfishness. The very thought of loving those things is odious to the sinful heart.
Later in this same chapter, the apostle writes, “God is love, and the one who abides in love abides in God, and God abides in him” (1 John 4:16)—again making the godly kind of love the mark of genuine faith.
Martyn Lloyd-Jones listed ten simple, practical ways of knowing whether we abide in love.  Lloyd-Jones, The Love of God, 150–53. I’ve paraphrased them here and added Scripture references to underscore each point:
Is there a loss of the sense that God is against me? (Romans 5:1; 8:31).
Is there a loss of craven fear of God, and a corresponding increase in godly fear? (cf. 1 John 4:18; Hebrews 12:28).
Do I sense the love of God for me? (1 John 4:16).
Do I know that my sins are forgiven? (Romans 4:7–8).
Do I have a sense of gratitude to God? (Colossians 2:6–7).
Do I have an increasing hatred for sin? (Romans 7:15–16).
Do I desire to please God and live a holy life? (John 14:21; 1 John 2:5–6).
Is there a desire to know God better and draw near to Him? (Philippians 3:10).
Is there a conscious regret that my love for Him is less than what it ought to be? (Philippians 1:9–10).
Is there a sense of delight in hearing about God and the things of God? (Psalm 1:1–2).
Suppose you fail those tests. How can you know the love of God? In Lloyd-Jones’s words, “You need not start traveling the mystic way, you need not try to work up feelings; there is only one thing for you to do: face God, see yourself and your sin, and see Christ as your Saviour.”  Lloyd-Jones, The Love of God, 153–54.
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