If you want the truth about Reformed Theology, and do not wish to misrepresent it,
you must always go back to the Westminster Confession of Faith, which clearly, and in great detail, spells out Reformed (Calvinistic) Theology (and has many sound teachings mixed in with a false gospel). So here is what you need to read and digest (regardless of Sproul or anyone else).
Chapter XVII
Of the Perseverance of the Saints
[Note: my comments are in red]
I. They, whom God has 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. [Note: there is no ambiguity here whatsoever]
II. This perseverance of the saints depends not upon their own free will, but upon the immutability of the decree of election, [Note: this is false, since it depends on the power of God ("kept by the power of God"), and the power of the indwelling Holy Spirit working within the saint] 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, [Note: this is true and biblical] and the nature of the covenant of grace: from all which arises also the certainty and infallibility thereof.
III. 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. [Note: all of this is true, and includes "the sin unto death"]