[Notes on the preposition en (ἐν, 1722)] - Vine's Expository Dictionary of New Testament Words
[Notes on the preposition en (ἐν, 1722)]En, "in," is the most common preposition. It has several meanings, e.g., "of place" (e.g., Heb_1:3, lit., "on the right hand," i.e., in the position), and time, e.g., in 1Th_2:19; 1Th_3:13; 1Jo_2:28, in each of which the phrase "at His coming" (inadequately so rendered, and lit., "in His Parousia") combines place and time; the noun, while denoting a period, also signifies a presence involving accompanying circumstances, e.g., 1Th_4:15.
Further consideration must here be confined to the instrumental use, often rendered "with" (though en in itself does not mean "with"), e.g., Mat_5:13, "wherewith" (lit., 'in what,' i.e., by what means) shall it be salted"; Mat_7:2, "with what measure ye mete." Sometimes the instrumental is associated with the locative significance (which indeed attaches to most of its uses), e.g., Luk_22:49, "shall we smite with the sword?" the smiting being viewed as located in the sword; so in Mat_26:52, "shall perish with the sword"; cf. Rev_2:16; Rev_6:8; Rev_13:10. In Mat_12:24, "by (marg., 'in') Beelzebub," indicates that the casting out is located in Beelzebub. Cf. Luk_1:51, "with His arm." In Heb_11:37, the statement "they were slain with the sword" is, lit., "they died by (en) slaughter of the sword." There is a noticeable change in Rom_12:21, from hupo, "by," to en, "with," in this instrumental and locative sense; the lit. rendering is "be not overcome by (hupo) evil, but overcome evil with (en) good," en expressing both means and circumstances. A very important instance of the instrumental en is in Rom_3:25, where the RV, "faith, by His blood," corrects the KJV, "faith in His blood," and the commas which the RV inserts are necessary. Thus the statement reads "whom God set forth to be a propitiation, through faith, by His blood." Christ is a propitiation, by means of His blood, i.e., His expiatory death. Faith is exercised in the living God, not in the blood, which provides the basis of faith.