Doesn't 1 John 5:11-13 prove "once saved, always saved"?
The Bible speaks of believers as having eternal life now (1 Jn 5:11-13). This proves "once saved, always saved."
Sometimes Scripture speaks of believers possessing eternal life now, but it also talks about it as something in the future.
When eternal life is described as a present possession of Christians, this is an example of what theologians call "realized eschatology." This means that Christians participate now in a limited way in what they will experience more fully in the Kingdom of God to come. It doesn't necessarily imply those who have so partaken of the Kingdom and its gifts can't be lost (Heb 3:14; 6:4-6).
The Bible speaks of eternal life as something future (Mt 19:29; Mk 9:43-47; Ti 1:2; 3:7; Jude 21) and based upon our fidelity, by God's grace, to his commandments (Mt 19:16-17; 25:46; Jn 3:36; Rom 2:6-10; 1 Tm 6:18-19; Jas 1:12; 2 Tm 2:12).