The most defining attribute of God seems hard to pinpoint. There are so many from which to choose.
Many would say it is His righteousness, His faithfulness, or His unchanging nature. Perhaps most would assume that God’s Holiness is His most defining characteristic. However, defining God in light of His Holiness is like defining apples as being apple-like. The fact that God is holy means that God is set apart, different from anything else. This does not help humanity to understand what he is like. It simply shows that He is different.
To understand God from the human vantage point is to see Him through another attribute: His love. God is love. This is the most defining attribute of God. It describes His relation to His creation. God’s love explains how a righteous God can forgive sinners. It shows how an all-sufficient God can desire worship and relationship. It reveals how an immutable God can suffer and long after the lost. It is the driving characteristic driving the Christmas story. It is why God became human in the incarnation. Jesus Christ in His life on Earth showed the world God’s most defining attribute. He showed His love in His ministry to the disadvantaged. It was His love that led Him to the cross to die for the sins of the world.
It is also God’s love that Jesus commanded His followers to practice amongst each other as their defining characteristic. God’s love is the attribute that, communicated among believers, makes them different from the world. It sets us apart.