"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)John is not saying that loving our brothers and sisters is a prerequisite to loving God, rather, that anyone who loves God will, by nature, also love their brothers and sisters. Love for one another is, in part, the fruit of loving God.
It is much easier for us to love those whom we can see; we love our families, our friends, our heroes but how does one love God? How does one love someone whom they cannot see, hear, or touch? How do we love an invisible God? Furthermore, how do we love one who is self-sufficient and who needs no love? Someone has everything they need and possesses infinite love within them, how do we love such a one? We love them by loving the things they love and care for. We love them by loving their creations and the works of their hands.
Jesus, speaking of the judgment at the end of the age and of the reward for those who loved Him, congratulates them saying, "For I was hungry, and you gave Me something to eat; I was thirsty, and you gave Me something to drink; I was a stranger, and you invited Me in; naked, and you clothed Me; I was sick, and you visited Me; I was in prison, and you came to Me." (Matthew 25:35-36) Yet, when the righteous failed to remember when they had done such things to Jesus, He said to them, "Truly I say to you, to the extent that you did it to one of these brothers of Mine, even the least of them, you did it to Me." (Matthew 25:40) Truly, we love God when we love those whom He created in His image.
When loving those who God has created, we must be careful not to only love those who can love us back, but to also love the unlovely and the unlovable. Jesus warns us, "If you love those who love you, what credit is that to you? For even sinners love those who love them. If you do good to those who do good to you, what credit is that to you? For even sinners do the same." (Luke 6:32-33) It is easy to love those who love us back and to love those from which we receive some benefit in return, but when we love the unlovely and unlovable because they are God's children, then we are loving God as well.
Herein lies the truth of what John is saying: we cannot separate the love of God from the love of people; they are one and the same. We cannot love God and, at the same time, ignore those whom He has created. One must lead to the other. If we say we love God but fail to love His people, then we have failed to understand love and the God who is love. Those who truly love God will also love those created by Him. This is an immutable truth.