This perseverance of the saints depends not upon their own free will, but upon the immutability of the decree of election, flowing from the free and unchangeable love of God the Father; Westminster Confession of Faith, Chapter 17:2.
Philippians 2:12–13 (NKJV)
The idea that man has free will, an idea sponsored by Pelagius, adapted by the Council of Trent, and emphasized by Arminius and Wesley, is totally inconsistent with the Biblical plan of salvation. It is also inconsistent with the sovereignty of God, with divine omniscience and omnipotence, with the necessity of regeneration by the Holy Spirit, and of course with the pervasive Scriptural teaching of predestination.
Dr. Gordon H. Clark. Philippians. (Hobbs: Trinity Foundation, 1996). Pp. 71-72.
(Ibid. P. 73).There is more to say on these two verses, for as yet "fear and trembling" has not been explained. Those who hold that regeneration is a result of humanly initiated faith and must be protected by further good works see in this fear a fear of eternal punishment and because of that fear they must tremble. If the state of regeneration were permanent, and if one could not possibly fall from grace, and if God were really going to complete his good work in us, there would be no place for fear. [Philippians 1:6] This Romish, Lutheran, Arminian position fails to take into account the fact that there are different objects of fear.
The Westminster Confession of Faith, on the other hand, says that perseverance does not depend on man's efforts or man's will. Instead, the WCF rightly emphasizes that God causes the elect to persevere apart from their will:
Chapter XVII: Of the Perseverance of the Saints
1. They, whom God hath accepted in His Beloved, effectually called, and sanctified by His Spirit, can neither totally nor finally fall away from the state of grace, but shall certainly persevere therein to the end, and be eternally saved. (Phil. 1:6, 2 Pet. 1:10, 1 John 3:9, 1 Pet. 1:5,9)
2. This perseverance of the saints depends not upon their own free will, but upon the immutability of the decree of election, flowing from the free and unchangeable love of God the Father; (2 Tim. 2:18–19, Jer. 31:3) upon the efficacy of the merit and intercession of Jesus Christ, (Heb. 10:10, 14, Heb. 13:20–21, Heb. 9:12–15, Rom. 8:33–39, John 17:11, 24, Luke 22:32, Heb. 7:25) the abiding of the Spirit, and of the seed of God within them, (John 14:16–17, 1 John 2:27, 1 John 3:9) and the nature of the covenant of grace: (Jer. 32:40) from all which ariseth also the certainty and infallibility thereof. (John 10:28, 2 Thess. 3:3, 1 John 2:19)
The Westminster Confession of Faith (Oak Harbor, WA: Logos Research Systems, Inc., 1996).
Addendum: It should be noted that the other objects of fear of which Dr. Clark speaks refers to the respect that Christians have for God as their Father and the discipline they will face from Him should they walk in disobedience. Fear does not mean fear of losing one's salvation but a godly fear and respect for God's sovereignty.