How do Christians know that they have a relationship with God?

Christians have two main ways of knowing they have a relationship with God. Both of these come as promises in the Bible.

The first promise shows that there is a spiritual component that must be experienced to be understood.

For you did not receive a spirit that makes you a slave again to fear, but you received the Spirit of sonship. And by him we cry, "Abba, Father." The Spirit himself testifies with our spirit that we are God's children. Now if we are children, then we are heirs—heirs of God and co-heirs with Christ, if indeed we share in his sufferings in order that we may also share in his glory (Romans 8:15-17 NIV).

From this, we see that the relationship with God is more than an acquaintance or friendship. It is the relationship between a father and a child. It is a relationship where the child can call out "Abba" or "Daddy". This is the most exciting part of Christianity—the promise that God is not your enemy, waiting for you to sin so he can punish you. He is your Father, and a better one than any human father.

But how do we know we have this relationship? Romans tells us that God's Spirit testifies with our spirits. When we believe that Jesus has forgiven us, turn from our sins, and yield our desires to God, he fills us with his Holy Spirit. This is a noticeable change in us, and it confirms that we have this relationship with God.

The second promise from the Bible is also important.

But the fruit of the Spirit is love, joy, peace, patience, kindness, goodness, faithfulness, gentleness and self-control. Against such things there is no law (Galatians 5:22-23 NIV).

Here we see that the Spirit, who confirms our relationship with God, also changes us. The Spirit produces fruit in our lives—good fruit, like love, joy, peace, patience, and so on. When the Spirit is in your life, and you begin to hand the controls of your life (your will) over to Him, you will start to see good fruit in your life. This is not something that is done by your own effort. It is done by God, as you are led by the Holy Spirit.

This is how a Christian knows he or she has a relationship with God.