1Jn 2:3-4  And by this we know that we have come to know him, if we keep his commandments. Whoever says "I know him" but does not keep his commandments is a liar, and the truth is not in him, 

In the next chapter Apostle John spells out what the commandments are: to believe in God’s son, Jesus Christ and to love one another. 
1Jn 3:22-24  and whatever we ask we receive from him, because we keep his commandments and do what pleases him. And this is his commandment, that we believe in the name of his Son Jesus Christ and love one another, just as he has commanded us. Whoever keeps his commandments abides in God, and God in him. And by this we know that he abides in us, by the Spirit whom he has given us. 

We also have the explicit answer from Jesus when a Pharisee asked him what is the greatest commandment and he answered: to love God is the first and to love people as we love ourselves. 
Mat 22:36-40  "Teacher, which is the great commandment in the Law?"  And he said to him, "You shall love the Lord your God with all your heart and with all your soul and with all your mind. This is the great and first commandment. And a second is like it: You shall love your neighbor as yourself. On these two commandments depend all the Law and the Prophets."

The Apostle John also said speaks clearly about the necessity to love our brethren:
1Jn 4:20-21  If anyone says, "I love God," and hates his brother, he is a liar; for he 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: whoever loves God must also love his brother. 

Jesus also added the necessity to love our enemies not just our neighbor:
Mat 5:43-44  "You have heard that it was said, 'You shall love your neighbor and hate your enemy.'  But I say to you, Love your enemies and pray for those who persecute you,

To summarize, if we think ourselves Christians we must love God in word and deed. The way we show that we love God is by obeying his commandments to love our brethren in our worldwide Christian family, to love our neighbors as ourselves and to love even our enemies. How can we attain to this emotional state in our hearts? Only by changing our characters and natural tendencies and letting ourselves be renewed through the power of the Holy Spirit, because let’s face it, it is not natural to love our enemies, and sometimes it can be hard even to love even our brethren we meet with every Sunday. We must learn to evaluate every situation from God’s point of view and discern what He would have us do, by the spirit of grace and humility and love.
If we do not speak and act out of love, we cannot be God’s ambassadors, and if we say we are, we are liars.