Hebrews 10:39

But we are not of them who draw back unto perdition
There is a drawing back which is not unto perdition; persons may be attended with much unbelief, may be very cold and indifferent to Gospel ordinances, may fall into great sins, and may greatly backslide, and yet be recovered, as David, Peter, and others: and there is a drawing back to perdition; when Christ is rejected as the alone Saviour; when he is not held to as the head; when false doctrines and damnable heresies are given into; and when men draw back, and never return, nor are they, nor can they be returned, and their apostasy is total, and final: but true believers do not, and cannot draw back in this sense; because they are held fast in the arms, and with the cords of everlasting love, are chosen of God unto salvation, are given unto Christ, and secured in him; they are redeemed and purchased by him; they are united to him, and built upon him; they are interested in his prayers and preparations, and are his jewels, and his portion; they are regenerated, sanctified, inhabited, and sealed by the Spirit of God, and have the promises and power of God, on their side.

But of them that believe to the saving of the soul;
or "of faith, to the salvation of the soul"; not of faith of miracles, nor of an historical faith; but of that faith, which is the faith of God's elect, is the gift of God, and the operation of his Spirit; by which a soul sees Christ, goes to him, lays holds on him, commits all to him, and expects all from him: this stands opposed to drawing back; for by faith a man lives, walks, and stands; and with this is connected the salvation of the soul, as opposed to perdition; not as though it is a cause of salvation, but as a means of God's appointing to receive the blessings of salvation, and which is entirely consistent with the grace of God; and since salvation and faith are inseparably connected together, so that he that has the one shall have the other, it follows, that true believers can never perish. The nature and excellency of this grace is largely treated of in the following chapter.