The greatest command in the Bible is to love the Lord your God with all your heart, mind, and strength. The second is to love your neighbor as yourself.
It is amazing to note that when I explain the 10 Commandments to people, and ask them about their love for God, almost always, the person says that they love God. I ask them a few questions to probe a little deeper. What I want to do is have them look at their own hearts and see that they really don't love God. I am talking mainly about people I have been explaining the Gospel to, whom I know by talking with them, that they don't understand the Gospel. But there is a gauge you have that can help you determine your love for God.
All the 10 Commandments are based on the two commandments above; Love God, and love your neighbor as yourself. The way you can tell if you love God is simple; Do I love my neighbor? Do I love those around me? Do I love those in my family and my friends? Do I love those who are ungrateful to me, even when I serve them? Do I love those on my street?
One reason this is hard for people to understand is because we don't have an accurate understanding of the word, 'Love'. We think of love as something that makes us happy. It serves us. We love ice-cream or Chinese food. We love chocolate and coffee. Those are things that serve us and we enjoy, but technically we don't 'love' them as the Bible defines love.
The Bible describes what love looks like. In 1 Corinthians 13, we find a true description of love. Some words that describe love are; patient, kind, rejoices in truth, believes all things, bears all things, endures all things, hopes for all things, and never ends. Some things that love is not; boastful, arrogant, envious, rude, insisting on its own way, irritable, rejoices at wrongdoing, resentful.
The Golden Rule is very good also, in helping us to understand about love. We treat others the way we want to be treated.
So, when we love others (even the unlovable) we are showing our love for God. Conversely, when we don't love others, it indicates our hearts do not love God. We can fool ourselves all we want, but the fruit of our lives indicates what is going on inside.