Acts 8:16

Translators appear not to understand ὑπῆρχον "to make beginning/take initiative" and seem to drop it or flatten it out of existence altogether. In Greek, “they have been plunged (baptized) in” would be expressed in the perfect passive (or middle-passive) indicative of βαπτίζω: ἐβαπτίσθησαν ἐν. This is not in the text, but is made up. 

What is written?

βεβαπτισμένοι ὑπῆρχον εἰς

"those who were dipped were taking intiative into"

There is no prepostion ἐν "in" but rather εἰς "into." 

ὑπῆρχον εἰς + acc./dative – Used with an accusative object and εἰς + accusative/dative to indicate proactive or contributive action. For example ὑ. εὐεργεσίας εἴς τινα or τινι is literally, “take the initiative in [doing] kindnesses to someone.” The accusative marks the benefit or service (εὐεργεσίας), and the dative (τινι) marks the recipient. (Demosthenes, 19.280; Aeschines, 2.26) The construction emphasizes intentional action on behalf of another, rather than mere existence of the favor.

(cf. LSJ ὑπάρχω A.4)