Thus says the Lord , the King of Israel and his Redeemer, the Lord of hosts: “I am the first and I am the last; besides me there is no god.
Isaiah 44:6 ESV
In this translation, a trinitarian understanding of God in this verse isn’t inconceivable- as the Lord, the King of Israel, and His Redeemer, the Lord of hosts, seem to be differentiated. Other translations seem more ambiguous about the difference. But however we read it, in the context of Scripture’s canon, we can see this is about the triune God.
Jesus was crucified by the Romans for apparent treason, because He claimed to be the King of the Jews- equating Himself with the Lord God here. And the Jewish leaders wanted Him crucified for blasphemy, because He claimed to be the Son of God. So He was either a fraud, or who He claimed to be!
Here, God describes Himself as the first and the last. In Revelation, Jesus does the same. He is the Alpha and Omega, the beginning and end, the A to Z, the “be all and end all”. Jesus equates Himself with God.
That last phrase, “besides me there is no God” could be contrued to be unitarian. But even within the verse we’ve seen plurality in God. God might be one, but Jesus and the Father are one, and one with the Holy Spirit too. God can only be Redeemer because the Father sent the Son to save us and the Spirit applies that work to us. All praise to the triune God for His wonderful redemption!
“God of the angel armies, our Redeemer, how we praise You, Father, Son and Holy Spirit, for our redemption! May we trust in You both now and forevermore. In Christ’s name, amen”