Thomas R. Schreiner, a defender of the Perseverance of the Saints, wrote that
Biblical writers recognized that
there were some within the church who were not truly believers. Paul writes in
1 Corinthians 11:19 that “There must, indeed, be factions among you, so that
those who are approved may be recognized among you.” The word “approved” (δοκιμοι)
signifies those who are truly saved in the church, and Paul implies that
there are “unapproved” (αδοκιμοι)
people in the church. The word “unapproved” (αδοκιμοι) in Paul always refers to
unbelievers (cf. 2 Corinthians 13:5, 6, 7; 2 Timothy 3:8; Titus 1:16).
Those who apostatize were not elected to salvation; they are the “unapproved,” who
never belonged to God from the beginning. (Thomas R. Schreiner, “Promises
of Preservation And Exhortations to Perseverance,” in Whomever He Wills: A Surprising
Display of Sovereign Mercy, ed. Matthew Barrett and Thomas J. Nettles [Cape
Carol, Fla.: Founders Press, 2012], 210, emphasis added)
The problem for Schreiner et al., is that Paul himself warned he
himself could become αδοκιμος
(“reprobate”) in 1 Cor 9:24-27, a term which Schreiner acknowledges refers to
someone who does not belong to God:
Do you not know that those who run
in a race all run, but only one receives the prize? Run in such a way that you
may win. Everyone who competes in the games exercises self-control in all
things. They then do it to receive a perishable wreath, but we an imperishable.
Therefore I run in such a way, as not without aim; I box in such a way, as not
beating the air; but I discipline my body and make it my slave, so that, after
I have preached to others, I myself will not be disqualified (αδοκιμος). (1 Cor
9:24-27 NASB)
For more on this pericope, see:
Craig Blomberg on 1 Corinthians 9:24-27 and the "Perseverance of the Saints" (cf. An Examination and Critique of the Theological Presuppositions Underlying Reformed Theology)