The Bible tells us, "Therefore if any man is in Christ, he is a new creature; the old things passed away; behold, new things have come" (2 Corinthians 5:17). A Christian is someone who has the Holy Spirit living inside his/her life (see 1 Corinthians 6:18-20). The Holy Spirit is God, the third person of the Trinity. When He comes to live within us, He changes us from the inside out. He begins working on us to make us like Jesus. This process is called sanctification as the Spirit begins to skim off the dross and impurities from our life. He works to make us holy as He is holy.
The acid test of true salvation is simply this: has there been a change in the way we act and react? Has there been a change in our attitude toward sin? Has there been a change in our desire to worship, pray, read the Bible, and gather with other Christians? If there is no change, there is no true relationship with Christ because He always brings a transformation to every life He inhabits.
Think of it this way. If I were to move into your home to live, is it conceivable that you would be oblivious to that fact? Absolutely not! You would know very quickly that another person is now under your roof, especially when I demanded the master bedroom! Things would change if I came to live in your home as the new master of the house. That is precisely what happens when Jesus comes to live inside of us by His Spirit. Things change ... and they change for the better. Paul said, "But the fruit of the Spirit is love, joy, peace, patience, kindness, goodness, faithfulness, gentleness, self-control; against such things there is no law" (Galatians 5:21-22).
Does a Christian still struggle with sin and selfishness? Yes. Each day, we must make the choice to yield the throne of our life to the control of the Holy Spirit. And, as we live under His Lordship, people notice that we are very different from the person we used to be. "The sweet aroma of the knowledge of Him" goes forth into every place we go (see 2 Corinthians 2:14). We are changed and changing. And it is wonderful to behold!