You first ask them about these scriptures found in the Old Testament to have a basis to agree on. If they agree that

God is the savior as the Bible teaches from the Old Testament then affirm what they believe, that Jehovah is the only God and the only savior, the redeemer.

Everything is to be asked by a question not by you telling them. You want them to read the Scripture fully, out loud

as they turn to it.

 “For I am the LORD your God, the Holy One of Israel, your Savior” (Isa.43:3).

Isaiah 43:11 “I, even I, am the LORD, And besides Me there

is no savior.”

Isaiah 49:26: “That I, the LORD, am your Savior,

And your Redeemer, the Mighty One of Jacob.”(Also Isaiah 60:16)

Isa. 48:17: “Thus says the LORD, your Redeemer, the Holy One of Israel: "I am the LORD your God.”

Hosea 13:4 “And you shall know no God but Me;

For there is no Savior besides Me.”  He says to the whole

world “Look to Me, and be saved, all you ends of the earth!

For I am God, and there is no other” (Isa.19:20)

When they agree, then you ask them again what do these scriptures mean in the New Testament?

 Have them read - 2 Peter 1:11: “for so an entrance will be supplied to you abundantly into the everlasting kingdom of our Lord and Savior Jesus Christ.”

Ask this question - In Titus 2:13 who is it that is called our great God and Savior?

“The Lord Jesus Christ our Savior” (Titus 1:4, 2:10, 3:6; 2 Tim.1:10)

Ask if these verses agree with the Old Testament.

If not, ask them to read...

Titus 3:4-6 “But when the kindness and the love of God our Savior toward man appeared.”

God says “And he shall send them a Savior” (Isa.19:20).

who was sent as the savior? Another other than the Father.

1 John 4:14: “That the Father sent the Son as the Savior of the world.”

Isa.63:8-9 explains to us what happened: “So he became their Savior. In all their affliction He was afflicted; and the Angel (messenger) of his presence saved them.” 

Who was this that was afflicted? When was God afflicted? 

Have them read Isa. 53:7 which says, “He was oppressed and He was afflicted, yet He opened not His mouth; he was led as a lamb to the slaughter.” 

If there is any doubt have them read  1 Tim. 4:10 “He (Jesus) is the Savior of all men.”

Have them read again Isaiah 43:11 “I, even I, am the LORD, And besides Me there  is no savior.”

If God says he is the only savior in the Old Testament and he sent his Son to be the savior in the New Testament, then Jesus is the savior and Jesus has to be God or he can’t be the savior.

Mary said “My spirit has rejoiced in God my Savior” (Lk.1:46-47). Joseph is told to call this son Jesus, ‘for he will save his people from their sins” (Mt.1:21). He is announced “For there is born to you this day in the city of David a Savior who is Christ the Lord” (Lk.2:11).

Jesus as a baby is brought into the temple by Mary, Simeon seeing him says, “My eyes have seen your salvation.” The Savior is a person.

If one does not believe Jesus is God then they cannot agree with Jude who writes “To God our Savior, who alone is wise, be glory and majesty, dominion and power, both now and forever. Amen” (Jude 1:25).

There is only one savior.