1John 4:7,8  Dear friends, let us love one another, for love comes from God. Everyone who loves has been born of God and knows God. Whoever does not love does not know God, because God is love.

John 13:35 "By this all men will know that you are my disciples, if you love one another."

It seems John reiterates this point for emphasis. Apart from the very fundamental truths of the faith, doctrinal correctness is not the primary measure of whether one is born of God. Rather the primary measure is love for fellow Christians. You'll find plenty of alleged Christians who zealously defend their particular denominational doctrine and make it the standard of salvation, but who also exercise a zealous illegitimate hatred for any Christian outside of their denomination. To the degree to which they behave in this manner, one can confidently say that such alleged Christians have not been born of God.

1Cor 8:3 the man who loves God is known by God.

1John 4:9,10 This is how God showed his love among us: He sent his one and only Son into the world that we might live through him. This is love: not that we loved God, but that he loved us and sent his Son as an atoning sacrifice for our sins.

Elsewhere it says, "God demonstrates his own love for us in this: While we were still sinners, Christ died for us." Rom 5:8 I think the reason he brings this up is that if we are commanded to love one another, we have to understand what is meant by "love". The way the world thinks of "love" is generally much different than what John is referring to here as "love". In fact the American Heritage Dictionary doesn't even contain the Biblical definition of love. It simply associates love with certain feelings or sexual desire. But as I said previously, Biblical love is not simply a "feeling" of affection. Rather it is the willful intention of meeting the real needs of others. As such, Biblical love is measurable by the degree to which you are meeting the real needs of those you allegedly "love". And this is where the cross serves as an example.

Notice in particular that the example of the cross is not one in which you love those who love you. Jesus said, "If you love those who love you, what credit is that to you? Even ‘sinners’ love those who love them." Luke 6:32 Nor is it an example of loving those who are good enough to be worthy of your love. Rather it is an example of loving sinners. If you feel other Christians are not good enough for you to love, then you're not following the example of the cross.

1John 4:11,12  Dear friends, since God so loved us, we also ought to love one another. No one has ever seen God; but if we love one another, God lives in us and his love is made complete in us.

There's an interesting fact about the word "so" here of which few may realize  impacts the meaning. "So", the Greek word "houtos", does not refer to the magnitude of God's love but to the manner of God's love. Likewise "houtos" is used in John 3:16 "For God so loved the world that he gave his one and only Son ..." The meaning is "For God in this manner love the world .." It does not mean "For God love the world so much"

So also here the idea is not that God love us so much that we should love one another so much. But rather since God loved us in this manner (the cross), in this manner we should love one another.  Which among other things means dealing with other people's sins. Likewise Paul writes, "Husbands, love your wives, just as Christ loved the church and gave himself up for her to make her holy, cleansing her by the washing with water through the word, and to present her to himself as a radiant church, without stain or wrinkle or any other blemish, but holy and blameless." Eph 5:25-27 But such an idea is unpopular in the post-modernistic feel good Christian community in which any confrontation with regards to sin or correction is treated as itself being sinful.

"No one has ever seen God, but God the One and Only, who is at the Father’s side, has made him known." John 1:18 And in particular we know God's love through his Son and what he did on the cross.

If we love others in that manner, then God's love for us has fulfilled its objective in our lives.