As the Westminster Confession says:
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. (6.095)
Of course, if the Colts prevail, there's always this passage:
Nevertheless they may, through the temptations of Satan and of the world, the prevalency of corruption remaining in them, and the neglect of the means of their preservation, fall into grievous sins; and for a time continue therein: whereby they incur God’s displeasure, and grieve his Holy Spirit; come to be deprived of some measure of their graces and comforts; have their hearts hardened, and their consciences wounded; hurt and scandalize others, and bring temporal judgments upon themselves. (6.096)

