Predestination is the eternal, sovereign, and unchanging purpose of God by which He has ordained all things for His own glory, specifically electing some to eternal life in Christ while leaving others to their just condemnation. Reformed traditions maintain that this doctrine is a source of great comfort and humility, emphasizing that salvation is entirely of grace.
Predestination is understood within the Reformed tradition as the eternal counsel of God's will. According to the 1689 LBCF Ch.3 §3, this decree encompasses the choice of some men and angels to eternal life through Jesus Christ to the praise of His glorious grace, while others are left to act in their sin to their just condemnation. As noted in the Westminster Shorter Catechism Q.7, God has foreordained whatsoever comes to pass for His own glory. This means that the number of the elect is certain and cannot be changed, as stated in the 1689 LBCF Ch.3 §4.
Rather than speculating on hidden decrees, the Christian is called to look to Christ as the 'looking glass' of our election, as emphasized in the Second Helvetic Confession Ch.X. Our election is not based on any merit of our own, but is found entirely in Christ, as the Apostle writes: Ephesians 1:4, 'even as he chose us in him before the foundation of the world, that we should be holy and without blemish before him in love.' Because we are more sinful than we dared believe, we find that our only hope is the grace of God revealed in the gospel. By trusting in Jesus, we find the surest testimony that we are inscribed in the Book of Life.