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