"So we know and rely on the love God has for us" from I John 4:16, NIV
The word translated 'rely' here (or 'believe' in the ESV) is the word for faith. According to my resident Greek scholar, it is written in the perfect tense (this is actually important!) which means that it is an action completed in the past with continuing effects. The idea is not just one of intellectual assent but of faith based on an experience of past trustworthiness. In other words, I don't merely believe in God's love, I have experienced it and know that it is reliable. Isn't that wonderful? The love of other people cannot be relied on. Friends may reject and abandon me. Even my husband may at times disappoint me. But God's love is reliable. He will never love me less. He will never misunderstand me. He will never wound me with thoughtless actions or careless words. He will never wrongly sacrifice me on the alter of some selfish agenda. I can rest in his love with full confidence. How great is my God!