The visible church of Christ is mixed; having both wheat and tares. It is the characteristic of God's people to persevere to the end because God preserves them (John 6:38-40, John 10:28-29, Romans 8:28-39). Those who fall away demonstrate that they had spurious faith. (See 1 John 2:19) The Holy Spirit quickens His people and since His seed abides in them He keeps them from falling away because they have been born again (1 John 3:19). He also preserves His own by disciplining us when we fail to judge ourselves, so that we won't be condemned along with the world (1 Cor 11:31-32). God also uses his word as a means of preserving us by calling us to persevere and giving us warnings of what will occur if we don't. The elect hear his voice in the word and take heed. The non-elect (tares) do not hear his voice and fall away.
"He will not forsake His saints. They are preserved forever." - Psalm 37:28
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Chapter XVII. Of the Perseverance of the Saints
Section I.–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.
Section II.–This perseverance of the saints depends, not upon their own freewill, but upon the immutability of the decree of election, flowing from the free and unchangeable love of God the Father; upon the efficacy of the merit and intercession of Jesus Christ; the abiding of the Spirit and of the seed of God within them; and the nature of the covenant of grace; from all which ariseth also the certainty and infallibility thereof.