Here is what Paul wrote:
"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" (Eph.1:4).
And here is how we are chosen:
"But we are bound to give thanks alway to God for you, brethren beloved of the Lord, because God hath from the beginning chosen you to salvation through sanctification of the Spirit and belief of the truth" (2 Thess.2:13).
In the Jamieson-Fausset-Brown Bible Commentary we see that the words "from the beginning" refers to "before the foundation of the world":
"from the beginning--"before the foundation of the world" (Eph 1:4; compare 1Co 2:7; 2Ti 1:9); in contrast to those that shall "worship the beast, whose names are not written in the book of life of the Lamb slain from the foundation of the world" (Re 13:8)" [emphasis added] (Jamieson-Fausset-Brown Bible Commentary, Commentary at 2 Thess. 2:13).
I'll tackle it from this passage, as all Paul's passages on this are the same in this - Ephesians 1:
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.
What Paul is asserting in all these passages is that; before the foundation of the world, God determined that He would choose those in His Son - who trust His Son - to be holy and without blame before God in love;
in His Son where
in He have made us accepted
in the Beloved -
in His Son.
This issue of being holy and without blame before Him in His Son - is the issue of our adoption as Sons - from children of God to Sons of God; where a child is recognized as an heir; with all its rights and privileges.
Its God's good pleasure that those who believe on His Son be placed into living union with His Son; accepted in Him; in the beloved, to the praise of the gory of His grace in us in His Son.
To the glory of this grace in us in His Son as it is witnessed in action by those principalities and powers in heavenly places that Ephesians 3 relates, are learning about this grace as it plays out in us.
As we "walk in" same, holy, or, "not conformed unto this world, but transformed by the renewing of our mind" by this information about all this, this information that is "according to the image of Him that created" this New Man we have been made a part of.
All this predestinated issue is God determined in eternity to do in His Son; in whom we have been accepted:
12. That we should be to the praise of his glory, who first trusted in Christ.
13. In whom ye also trusted, after that ye heard the word of truth, the gospel of your salvation: in whom also after that ye believed, ye were sealed with that holy Spirit of promise,
14. Which is the earnest of our inheritance until the redemption of the purchased possession, unto the praise of his glory.
Heard the gospel of your salvation, trusted, or believed, were sealed
in Christ with that Holy Spirit of Promise, which is the earnest of our inheritance, until the redemption of the purchased possession, unto the praise of his glory.