What do you tell a person that is not a Christian? What if a person does not know whether he is born again? Your friend thinks he is a Christian, but you know better. What do you tell them?
There are some "life and death" statements in the Bible that should get your attention. "Unless you believe that I am He, you shall die in your sins" (John 8:24). Another is "...Jesus shall be revealed from heaven with His mighty angels in flaming fire, dealing out retribution to those who do not know God and to those who do not obey the gospel of our Lord Jesus. And these will pay the penalty of eternal destruction, away from the presence of the Lord and from the glory of His power." (2 Thessalonians 1:7b-9).
It is imperative to believe in and obey the Gospel. However, there is sometimes a confusion concerning the Gospel that leaves it untold. What is the Gospel? Paul tells what the Gospel is: "Now I make known to you brethren, the gospel … by which also you are saved, … For I delivered to you as of first importance what I also received, that Christ died for our sins according to the Scriptures, and that He was buried, and that He was raised on the third day according to the Scriptures. (I Corinthians 15:1-4)"
Notice that the Gospel is what saves. Notice also what the Gospel of: Jesus died for our sins, was buried, and was raised on the third day. That is the heart of the Gospel message. Paul even stated that he delivered "as of first importance" these facts.
The Gospel is not the "one true church", it is not "A Cappella music", it is not even "baptism". Too often we have a tendency to jump into those discussions without ever getting to the point. Jumping into Baptism without sharing the Gospel is getting the cart in front of the horse. Those are not the things the Paul said were "as of first importance". Don't misunderstand, the Bible is clear about the necessity of baptism. However, the object of our faith is not baptism, but in the resurrected Lord Jesus. When Peter preached the first Gospel sermon in Acts 2, he preached Jesus as the promised Messiah. It wasn't until the people were "pierced to the heart" and were ready to do something about it that Peter preached Baptism. To preach Baptism to wash away your sins without first preaching the blood of Jesus which washes away your sins is to reduce the Gospel to a system of works where you merit your own salvation, which is useless. power is not in the water, but in the blood. (I Peter 1:18-19)
So don't get the cart before the horse. Stick to the Point: The Gospel. When speaking to someone that is not yet a born-again Christian, determine to know nothing except "Jesus Christ, and Him crucified" (I Corinthians 2:2). Decide to open your mouth and preach "Jesus to him" (Acts 8:35). Lead them not to a religious issue, but to the "Lord" and "Savior" Jesus Christ.