A lot of people, including myself, tend to associate kindness with being nice. When we introduce ourselves to someone who had just moved into the neighbor many would think that is an act of kindness by being nice. When we give a someone a ride when their car breaks down, someone might say, "How nice of you to do that" as a way of saying they were being kind.
According to Wikipedia, kindness is defined as, "a behavior marked by ethical characteristics, a pleasant disposition, and concern for others. It is known as a virtue, and recognized as a value in many cultures and religions." Many religions teach about their people being kind to one another. The Bible says that the work of the Spirit as we abide in Christ is being kind but it many not be the same way as being nice.
Don't miss understand me, being nice is important to some extent, but kindness is more than just being a friend. A study note from the ESV Study Bible say kindness is "showing goodness, generosity, and sympathy toward others." Jerry Bridges wrote:
Kindness is a sincere desire for the happiness of others...Apart from God's grace, most of us naturally tend to be concerned about our responsibilities, our problems, our plans. But the person who has grown in the grace of kindness has expanded his thinking outside of himself and his interests and has developed a genuine interest in the happiness and well-being of those around him.
When we are kind to someone, we think of what is best for them. Acts of kindness can come in many forms. One way is physical such as repairing an elderly lady's sink for no charge or babysitting a newborn to give his/her parents a night to themselves. Another way is spiritual by speaking a word of encouragement or praying for them.
The Holy Spirit works in us to be kind to another because we have been shown great kindness from the Father:
But God, being rich in mercy, because of the great love with which he loved us, even when we were dead in our trespasses, made us alive together with Christ—by grace you have been saved—and raised us up with him and seated us with him in the heavenly places in Christ Jesus, so that in the coming ages he might show the immeasurable riches of his grace in kindness toward us in Christ Jesus (Ephesians 2:4-7).
While we were enemies of God, He showed us His kindness by sending Jesus to live on our behalf and died in our place for our sins. God shows us His kindness by not giving us what we deserve, which is wrath, and give us what we don't deserve, which is eternal life in Christ. Romans 2:4 says that God's kindness leads us to repentance. That is not only true when we first believe but also as we continue on in this journey while we sojourn on this earth. The work of the Spirit is show that same kindness to everyone, not just believers.