I have been witnessing to a friend who is a lesbian. She feels that since she believes in Jesus she is saved, but I have a problem with that because Satan and demons believe but we know they aren’t saved. Every time Jesus healed someone we know they believed in Him because they had faith He would heal them, yet Jesus told them to go sin no more. Repentance is a big part of salvation and yet some people don’t believe you have to repent to go to heaven. They say John 3:16 says plainly that he that believes in him shall have eternal life. What about repenting?
You’ve touched on several critical misunderstandings of Scripture, so let’s take them one at a time.
First, James 2:19 says, “You believe there is one God. Good! Even the demons believe that, and shudder.” Satan and his demons believe Jesus exists because they’ve seen Him. But neither this verse nor any other ever hints that they believe in him. And besides, Jesus became a man to save mankind. There’s no Scripture to support the idea of angels or demons being saved. If by saying she believes in Jesus, your friend means she believes Jesus died for her sins and rose again, then according to 1 Cor. 15:1-4 she is saved.
Second, Jesus did not include the general commandment to “sin no more” in all His healing miracles. He said it once in connection with a healing (John 5:14) and once in connection with forgiveness of the sin of adultery (John 8:11). In both cases He was speaking of a specific sin. We know this because if ceasing all sinful behavior was a condition of salvation, none us would be saved.
Third, the Greek word translated repentance in the New Testament means “to change one’s mind”. The reason it’s a big part of salvation is in order to be saved we first have to change our mind and accept the fact that we’re sinners. Then we have to change our mind and believe that Jesus died for our sins and rose again. Therefore, someone who is born again has already repented.
Behavioral changes happen after we’re saved as the Holy Spirit teaches us about living a life pleasing to God. But according to Romans 7:14-25 as long as we have a sin nature we will never completely stop sinning.
Finally, John 3:16 is only one among numerous verses that make salvation conditional upon belief, but there are none that make salvation conditional upon behavior.