How could the Israelites be fiercely monotheistic and yet refer to their God using the plural word Elohim?

First, this cannot be explained away as a “royal plural” or a “plural of majesty.” Hebrew Scriptures never used a first-person plural to refer to any speaker other than God himself (e.g., Genesis 1:26).

Furthermore, while the Bible from Genesis to Revelation reveals that God is one in nature or essence (Deuteronomy 6:4; Isaiah 43:10; Ephesians 4:6), it also reveals that this one God eternally exists in three distinct Persons: Father, Son, and Holy Spirit (1 Corinthians 8:6; Hebrews 1:8; Acts 5:3–4). Thus, the plural ending of Elohim points to a plurality of Persons, not to a plurality of gods.

Finally, although Elohim is certainly suggestive of the Trinity, the word alone is not sufficient to prove the Trinity. Instead of relying on a singular grammatical construction, Christians must be equipped to demonstrate that the one God revealed in Scripture exists in three Persons who are eternally distinct.

Hear, O Israel: The Lord our God, the Lord is one.
Deuteronomy 6:4


For further study, see Robert Letham, The Holy Trinity: In Scripture, History, Theology, and Worship (Phillipsburg, NJ: P&R Publishing, 2004).