I think that
Ephesians 1:3–6 tell what reformers believed and taught.
3 Blessed be the God and Father of our Lord Jesus Christ, who has blessed us with every spiritual blessing in the heavenly places in Christ, 4 just as He chose us in Him before the foundation of the world, that we should be holy and without blame before Him in love, 5 having predestined us to adoption as sons by Jesus Christ to Himself,
according to the good pleasure of His will, 6 to the praise of the glory of His grace, by which He made us accepted in the Beloved.
God is glorified each time He takes a poor wretched sinner who has absolutely no power to save himself, and adopts him as a son. And, please note that He did this before we existed. Therefore, no boasting allowed!
Why He picked certain people is not explained, only that it was according to the good pleasure of His will....... God's plan for His creation will not fail.