I can understand that your theological position says what it does.
However, what you are doing is attempting to rewrite the Bible according to your theology. And to be perfectly honest, if there ever was a verse written exactly as you posted, then I would be forced to believe as you do.
In reality, we must make our theology to be solely dependent upon what Scripture says, and not what we wish it to say
Please look at the actual words, in the context, and in the word order as well as in context, which is not clear in the OP.
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Ephesians 1: 3 Blessed be the God and Father of our Lord Jesus Christ, who hath blessed us with all spiritual blessings in heavenly places in Christ:
4 According as he hath chosen us in him before the foundation of the world, that we should be holy and without blame before him in love:
5 Having predestinated us unto the adoption of children by Jesus Christ to himself, according to the good pleasure of his will,
6 To the praise of the glory of his grace, wherein he hath made us accepted in the beloved.
7 In whom we have redemption through his blood, the forgiveness of sins, according to the riches of his grace;
8 Wherein he hath abounded toward us in all wisdom and prudence
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Verse 3 begins with a doxology, which is a fancy name for a hymn of praise to God. "Praise God, from Whom all blessings flow..." So why is it there? Well the answer is that it is the opening for a letter to the Ephesians, and Paul wants to remind them of the wonderful salvation that God has given to them. After all He is the Author of the plan of our Salvation.
Specifically, this the spiritual reason for the doxology: who hath blessed us with all spiritual blessings in heavenly places in Christ: because of Jesus, and His Atonement, God has blessed us with blessings that are in heaven, (which we cannot see yet), and those are IN Christ. Were it not for Him, we would not have any of those things.
Verse 4 begins to make a list of the specifics for that doxology.
"According as he hath chosen us in him before the foundation of the world..."
The word "according" means "to be in full agreement with...", or "following the rules of (something or someone)" Got that, so far?
Therefore the "rule book" where Jesus blessed us with heavenly blessings necessarily lies in the next words: "He hath chosen US in Him..." You may not be aware of it, but the phrase "I will be their God" is repeated about 210 times in the Old Testament (my software told me this) Sometimes, it is followed with the phrase, "...and they shall be my people."
What is significant about this is one thing:
God chooses.
Nothing else is a close second, excepting, perhaps the reciprocal of the fact that God chooses, is that His Chosen also become His people.
Please notice the time line here. Of course there is no time for any other created creature. Have you ever seen a dog or cat wearing a watch? (Yes, hyperbole) But sometimes that is good to help make the point.
But God the Father chose Us who are Christians (and will become Christians) IN CHRIST. Please do not ask me to explain that prepositional phrase; I can't. All I know is that is too wonderful for me to fully understand; nevertheless it is true.
This happened before mommy and daddy had a certain gleam in their eyes, and I am the result of that gleam. According to Scripture, it happened BEFORE THE CREATION OF THE WORLD:
It happened WAY before God created the Universe, and it happened outside of time itself.
Please understand, I am NOT trying to make you into a "Calvinist". Instead, I am attempting to to put Scripture ahead of theology, and be a "Scripturist" before anything else, K? Look at the verses I cited in several different versions. Bible Gateway is a good place to do that. enter "Ephesians 1:4 " in your search parameters, and then look at other versions . If I recall correctly, that is an option on the page giving you the verse.
Thy using the phrase "I will be their God", in the search function, and find out what happens
Please get back to me when you do that.