If you speak to someone who claims to know God and you are not certain of their salvation, ask them if they have been born again(John 3:1-7). If they are not sure (1 John 5:10) or they say that they haven't, here is how you can bring focus to its importance. Tell them that the difference between believing in God and being born again is like the difference between believing in a parachute and putting it on. There's a big difference when you jump from the plane. Then say, Do you know what convinced me that I had to be born again? It was the 10 Commandments. Then take them through the spiritual nature of the Law, which brings the knowledge of sin (Romans 7:7). 1 John 5:1- Evidence Bible