And how does one recognize a "true" cherry-picking believer from an "untrue" one?
According to Mark (16:17), true believers will "cast out demons" with Jesus' name, and "speak with new tongues."
Likewise, there will be "signs" (16:18) of the believers to include picking up serpents, drinking any deadly poison without getting hurt, and curing the sick by laying hands on the them.
That sounds great, except (1) Mark's Great Commission does not even resemble Matthews' (who otherwise borrows from Mark not only verses but whole paragraphs verbatim!); when it comes to the Great Commission, the two are like night and day. (2) Many, more like all of them, who show these "signs" are clowns and impostors. And (3) that part of Mark's Gospel (16:9-20) where these verses are found is missing from all older manuscripts and is a latter-day addition to the "word of God."
Every believer is indeed a priest to God, through our intercessor and High Priest Christ Jesus.