Perseverance of Saints
The perseverance of the saints means that all those who are truly born again will be kept by God’s power and will persevere as Christians until the end of their lives, and that only those who persevere until the end have been truly born again.
Wayne Grudem from Systematic Theology (pg. 788)
If our religion be of our own getting or making, it will perish; and the sooner it goes, the better; but if our religion is a matter of God's giving, we know that He shall never take back what He gives, and that, if He has commenced to work in us by His grace, He will never leave it unfinished.
For non-reformed theologies..."at the end of the day, the security of the believer finally rests with the believer. For those in the opposite camp [Reformed], the security of the believer finally rests with God -- and that, I suggest, rightly taught, draws the believer back to God himself, to trust in God, to a renewed faith that is of a piece with trusting him in the first place."
When we speak of “once saved, always saved,” we are not taking into account the full scope of salvation. We have been saved (justification), was are being saved (sanctified), and we will one day be saved (glorified). You cannot claim to have been “saved” (justified) unless you are being sanctified. Jesus Christ is Savior and Lord.
Michael Horton from Putting the Amazing Back into Grace (pg. 171)
Those who think they can lose their salvation are not trusting in Jesus Christ alone for salvation but partly trusting in their own righteousness. Such teaching would have you believe that the sacrifice of Jesus was insufficient to make his people perfect forever (Heb 10:14) or save them to the uttermost (Heb 7:25) [i.e. that Jesus is not enough]. ... such that, in addition to what Christ did, they must join their own ability to persevere to the end in order to maintain their own just standing before God. The book of Hebrews and Galatians severely warns against this and all teaching that rejects the sufficiency of Christ alone (Heb 6 & 10; Gal 3:3). Such persons think Christ made the down payment on their salvation but that they have to keep up the monthly installments (so to speak). I total misapprehension of the meaning of paid in full."