Reformed doctrine on election and predestination is Biblical. It’s what we would call observable.
Ephesians 1: NASB
3Blessed
be the God and Father of our Lord Jesus Christ, who has blessed us with every spiritual blessing in the heavenly
places in Christ,
4just as He chose us in Him before the foundation of the world, that we would be holy and blameless before Him. In love
5He predestined us to adoption as sons through Jesus Christ to Himself, according to the kind intention of His will,
6to the praise of the glory of His grace, which He freely bestowed on us in the Beloved.
7In Him we have redemption through His blood, the forgiveness of our trespasses, according to the riches of His grace
8which He lavished on us. In all wisdom and insight
9He made known to us the mystery of His will, according to His kind intention which He purposed in Him
10with a view to an administration suitable to the fullness of the times,
that is, the summing up of all things in Christ, things in the heavens and things on the earth. In Him
11also we have obtained an inheritance, having been predestined according to His purpose who works all things after the counsel of His will,
12to the end that we who were the first to hope in Christ would be to the praise of His glory.
13In Him, you also, after listening to the message of truth, the gospel of your salvation—having also believed, you were sealed in Him with the Holy Spirit of promise,
14who is given as a pledge of our inheritance, with a view to the redemption of
God’s own possession, to the praise of His glory.