7 But if we walk in the light, as he is in the light, we have fellowship with one another, and the blood of Jesus his Son cleanses us from all sin. 1 John 17:–ESV)
John alternates away from falsehood and into truth. My Muslim friend would not have had the difficulty he did had the Christ-claiming ophthalmologist lived as Jesus did.
Let me diverge into fellowship a bit. I have found that I can be more at ease around an observant Muslim than I can around many men and women that comprise my immediate connections. Why? The adherence to specific standards of behavior and morality puts us onto a particular plane. Each of us lives with the perspective that includes God. We understand that God sees what we do and has opinions about those actions and activities. This viewpoint puts each of us into a certain rapport with one another. That is called fellowship.
The fellowship breaks down when we move to more foundational levels of course, but in choices and lifestyles, we can have commonality.
If we extrapolate these concepts to men and women who adhere to both the style of Christ and allegiance to him, then we see more plainly what John wrote of here in verse 7.
Christians are not perfect
Thoughtful people are not under the impression that Christians are perfect. We all falter in many ways, and those falterings must never be excused or be considered life like Christ. Sin still separates us from fellowship with God, Jesus, and our fellow man. But the blood of Jesus does cleanse us. So we are to keep our eyes on Christ, live like him with all our might, but when we trip, go to him with all of our confession. Then we will be brought back into right person-person fellowship as well as proper God-person fellowship.
8 If we say we have no sin, we deceive ourselves, and the truth is not in us. (1 John 1:8–ESV)
Here is another pothole of humanity. Some people in their efforts toward godliness will decide that they have arrived at a place of perfection. In Christian circles, the term for this is sinless perfection. Others have studied this and its implications in far greater depth than I have. One such man is the late R.C. Sproul. Check this link to a page at the Ligonier Ministries for a more rounded discussion. Other words that may be related to this concept include the following: second blessing, eternal perfection. It seems that the Nazarene church, some branches of the Wesleyan church and holiness churches engage this view.
I do not wish to head too thoroughly into the teachings and rebuttals of this false teaching. It is more crucial for you to have John’s words as the groundwork for your recognition this heresy. To counter it will require a further study of this verse and other relevant passages such as Romans 7.