knowledge: a grasp of what is true, whether held in the mind, lived out, or given by God.
The plain truth about this word is that it means knowledge, and almost nothing more specific than that until you read the sentence it sits in. It is the ordinary Greek word for knowing something, and outside the Bible it could mean anything from a scholar's understanding to a clerk's official report. So when New Testament writers reach for it, they are not pulling a loaded religious term off the shelf; they are using a flexible everyday word, and the surrounding words tell you what kind of knowing is meant.
That flexibility is why the same word can sit at opposite ends of the same letter. In one place Paul thanks God that the Corinthians are rich in knowledge; a few chapters later he warns that knowledge by itself only puffs people up while love builds them up. The word has not changed meaning. What changed is whether the knowledge is held with love or without it. Paul is never against knowing things; he claims this very gift for himself. What he attacks is knowledge cut loose from love and from the cross.
Across the New Testament the word ranges from God's own boundless knowing of his plans, to the saving recognition of who Jesus is, to a Spirit-given gift exercised in the church, to a status-symbol that the proud wield over weaker believers, to the false "knowledge" peddled by teachers the church should avoid. One word covers all of it, taking its color each time from what is being known, by whom, and for what end.
The single most useful caution is to resist the temptation to make this word carry one hidden deep meaning. It does not secretly mean a warm, relational, experiential knowing as opposed to cold head-knowledge, nor does it smuggle in a mystical or heretical sense. Where a passage means relational or saving knowing, the passage says so; the word itself is just the common word for knowledge.