What is the greatest evidence of your faith? Is it outward piety—how often you pray, go to church, read your Bible, and perform other overtly “Christian” activities? Or perhaps we ought to look for experiential evidence—do God and His Word compel an emotional response from you, or is it simply that you “feel” the Spirit at work in you?
No, the best proof of your faith is not found in how you act or how you feel, but in how you love. Love is the greatest evidence that a person knows God: “Beloved, let us love one another, for love is from God; and everyone who loves is born of God and knows God. The one who does not love does not know God, for God is love” (1 John 4:7–8).
Those verses point out the unmistakable connection between loving and being loved by God. Conversely, a lack of love indicates a lack of knowing God. I’m not talking about modern sentimentalized notions of love. The love that flows out of a regenerate heart reflects the pure love that God pours into the hearts of those He regenerates (Romans 5:5).
The Creator of the universe has, in His infinite wisdom, chosen to use the sinners He saves to reflect His loving nature. God has poured out His saving love on His elect. That in turn produces love among His people—a love which accomplishes three critical purposes in our fallen world.
Love Testifies to an Unbelieving World
When God’s love shines through our lives it proves to an unbelieving world that we belong to Him. Jesus affirmed this when He said: “By this all men will know that you are My disciples, if you have love for one another” (John 13:35). Our love for fellow Christians is a litmus test that separates authentic disciples from phony pretenders.
John MacArthur adds:
The church may be orthodox in its doctrine and vigorous in its proclamation of the truth, but that will not persuade unbelievers unless believers love each other. In fact, Jesus gave the world the right to judge whether or not someone is a Christian based on whether or not that person sincerely loves other Christians.  John MacArthur, The MacArthur New Testament Commentary: John 12–21 (Chicago: Moody Press, 2008) 91.
We should examine ourselves and our relationships with other Christians in light of this truth. What message are we sending the unbelieving observers of our lives?
Love Brings Assurance of Salvation
Joining a church won’t save you, but it might indicate whether you are saved. Jesus’ commandment to “love one another, just as I have loved you” (John 15:12) finds its fullest expression in the assembly of believers.
The importance Jesus placed on loving our brothers and sisters in Christ explains why we must be committed to the local church, and why “online churches” will always be inadequate. Genuine Christianity is proven through face-to-face interaction with other Christians—both those who are easy to love and those who are not so easy to love. What better way to grow in conformity to Christ than by loving sinners who are also in the process of sanctification?
Conversely, a failure to love our brethren exposes our lack of love for God:
If someone says, “I love God,” and hates his brother, he is a liar; for the one who does not love his brother whom he has seen, cannot love God whom he has not seen. And this commandment we have from Him, that the one who loves God should love his brother also. (1 John 4:20–21)
A genuine love for God and His church are the marks of authentic Christianity. Because, as John MacArthur rightly points out, that ability to love is the outward fruit of God’s inner transforming work:
All true believers have this love; and all who have it are true believers.
This kind of love cannot be conjured up by the human will. It is wrought in the hearts of believers by God Himself. “We love, because He first loved us” (1 John 4:19). Love for God and love for fellow believers is an inevitable result of the new birth, by which we “become partakers of the divine nature” (2 Peter 1:4). Just as it is God’s nature to love, love is characteristic of His true children. “The love of God has been poured out within our hearts through the Holy Spirit who was given to us” (Romans 5:5).
Godly love, therefore, is one of the most important tests of the reality of one’s faith.  John MacArthur, The God Who Loves (Nashville: W Publishing Group, 2001) 32.
Love Reflects God’s Love
The proper motive for human love is divine love. “We love, because He first loved us” (1 John 4:19). And while our immediate priority is toward our fellow believers, our responsibility to love doesn’t end there:
The Lord’s command to love extends beyond the church to embrace all people. Paul’s prayer for the Thessalonians was that they would “increase and abound in love for one another, and for all people” (1 Thessalonians 3:12). He exhorted the Galatians to “do good to all people, and especially to those who are of the household of the faith” (Galatians 6:10).  The MacArthur New Testament Commentary: John 12–21, 91.
Having said that, we must not confuse the love we show for others—especially unrepentant sinners—as evangelism. Loving people is not the gospel, nor is it an alternative to the gospel. Rather, it motivates and adorns the gospel that we preach.
We preach to the lost because we love them and don’t want them to perish—and if we are truly motivated by our love for the lost, unbelievers can usually sense it. Moreover we preach the gospel because we love God and desire His glorification through the proclamation of His great Name.
We cannot replicate God’s greatest acts of love—especially the cross. But we can reflect the love that He has poured out in our own hearts (Romans 5:5) by loving people in ways that point them to our great and loving God.