A changed life comes with being born again. Your existence here cannot help but be changed when your life is born from above. That change is a living testimony to salvation, but a changed life is insufficient as a means of evangelism.
I can introduce you to people who have joined the military and their lives have been changed from drugs and sloth to discipline and responsibility. You can meet those who have lost weight or stopped a long habit of cigarette smoking through the wonder of hypnosis. Any cult member can claim a changed life. A person’s life can be changed in many ways – all apart from a relationship with Jesus; so a changed life isn’t a testimony to others that you’re saved.
It is, however, a testimony to YOU. When you “know that you know that you know” that your life is different from the heart outward – not because of discipline or a mind-trick or some drug, but a first-hand encounter with the risen Christ – it is a testimony to YOU that Jesus is real and your salvation is true.
Therefore, brethren, be even more diligent to make your call and election sure, for if you do these things you will never stumble (2 Peter 1:10).