We're taught to assume the best about people, to give the benefit of the doubt. This is generally good advice. However, this is horrible advice to follow when dealing with a narcissist.
When dealing with a narcissist, it is better to assume they are interacting with you with the worst motivation. This will feel counterintuitive, especially when you tend to see the world in an idealistic way, expecting that people think in the same loving, considerate, thoughtful, sensitive way you do. It is a grave mistake to believe everyone thinks the way you do.
An understandable error, it is a significantly consequential mistake to believe that because someone calls themself a Christian that they are living a Christlike life, ignoring the Bible's warning about wolves in sheep's clothing. (Matthew 7:15) Narcissists who hide behind a spiritual persona (and many of them do) are the pure definition of a such a deception.
Make no mistake, you will encounter individuals who may think this approach is too cynical and unchristlike. Always consider the source when receiving advice. You can be assured that no survivor of narcissistic abuse will advise you differently. More than likely, they will tell you that the sooner you adjust your mindset in this area when it comes to interacting with a narcissist, the better.
One of the primary reasons that expecting the worst is the best approach is because their envious, immature, retaliative, selfish spirit leads narcissists to always prioritize their needs and desires while exploiting yours for their benefit (and in many instances your shame, also).
Assume the worst when dealing with a narcissist and then be surprised when the outcome appears to be acceptable. Even in this instance, however, your guard should never be lowered when dealing with this individual. Understand that every move on their part is strategic and made to benefit them in some way.
This simple, yet effective mindset adjustment, will help to provide stability in the sea of crazymaking, exhausting interactions with a narcissist. It will also help you to follow the Bible's admonition to be, "wise as a serpent and gentle as a dove." (Matthew 10:16)
Lord, please give my friend discernment when dealing with the narcissist in their life. Renew their mind so that Your wisdom can prevail and unnecessary suffering can be avoided. In Jesus' name, I pray. Amen.