As a believer, you can be eternally secure of God’s preservation because there is no sin that has not already been dealt with by Christ. Before Jesus we were enemies of God, but Christ interceded, put Himself, as the accused in our place, and through His death He destroyed this hostility towards God, and gave us peace between God and ourselves. Can we fall from the Lord’s hand? No, certainly not, God is faithful, mercifully strengthening His children in His grace and powerfully preserving us in this, His grace, to the end.

(Canons of Dort - Chapter 5)