Unlike English, Greek is a heavily (or highly) inflected language.1 In English, one could say, “One Lord, one faith, one baptism,” and the adjective “one” does not change spelling according to the noun it modifies.
In Greek, on the other hand, the typical adjective will decline2 according to:3
Since adjectives must agree with the nouns (or pronouns) they modify,4 then nouns also decline in the same manner (excluding degree).
The English adjective “one” is translated from the Greek word εἷς, which may be transliterated into English as heis.
This word εἷς is known as the λήμμα (lemma), or the dictionary/lexical form of the word. The lemma of an adjective is its form declined in the nominative case, singular number, masculine gender, and positive degree.
Since εἷς is an adjective, when you find its entry in a lexicon, the lexicon will give you its declension for the masculine, feminine, and neuter genders, all in nominative case, singular number.
With respect to Eph. 4:4–6, the lemma εἷς modifies nouns of various genders, hence the different declensions. Nevertheless, all possess the same general meaning: “one.”
1 Chadwick, Ch. 4, p. 35
2 To decline is to inflect a noun, pronoun, or adjective; to conjugate is to inflect a verb.
3 Because εἷς is inherently singular—after all, it means “one”—it does not decline according to number, unlike other adjectives. Also, being a numeral, it does not decline according to degree (i.e., it is not used as a comparative or superlative).
4 Mounce, p. 102
Mounce, William D. Basics of Biblical Greek. 2nd ed. Grand Rapids: Zondervan, 2003.
Thornill, A. Chadwick. Greek for Everyone: Introductory Greek for Bible Study and Application. Grand Rapids: Baker Books, 2016.