“Phenomenological Believer” describes a person who gives every [appearance] of being a “Christian” but in reality isn’t one. There are phenomenological believers in the Body of Christ, which is why warning passages such as [2 Peter 2] uses “Christian language” to describe these phenomenological believers.
In case you’re wondering, we do use phenomenological language today (cf. Psalms 104:19) when we say “Sunrise” or “Sunset” (for example), even though the Sun is stationary. Yet when we observe with our eyes, it gives the [appearance] of being in motion. The writers of the NT were inspired and could speak with precision, but sometimes spoke phenomenologically. This is needed because it sets an example for us since we’re not inspired and could not speak with divine authority. When it comes to dealing with “Christians” in general, take the advice of Conrad Murrell:
“Some people are obviously lost. Others are manifestly saints of God. But there are multitudes of marginal cases in which it is impossible for mortal man to discern or detect what their state in grace may be outside of special divine revelation. When a person is obviously a child of the devil, he ought to be treated and witnessed to as such. When a person is an obvious child of God, he ought to be received as a brother. When we have no definite proof either way, we are to receive one who professes to be a Christian as one until he gives reason to believe otherwise. And then we are not bound to make conclusions, but to warn him to forsake the error of his way and to give diligence to make his calling and election sure.”
No minister or Christian today can know the hearts of men fully. This is one of the reasons why the Parable of the Four Soils and many warnings, admonitions, and exhortations are given. The key to understanding apostasy, or any kind of falling away, lies in the Parable of the Four Soils: “And He said to them, ‘Do you not understand this parable [the four soils]? How then will you understand all the parables?’” (Mark 4:13)