"[The sinner] is bought by Christ and is redeemed and saved at this point. This grace however, does not guarantee ultimate salvation. There is still a race to be run by grace through faith. Grace is the means by which the salvation which has been granted salvation might be sustained until the end of his life, for he who endures to the end will be he who is saved. This grace is that which causes the believer both to will and to do of God's good pleasure, resulting in the believer's justification, which will be reaffirmed again and again as the believer continues to exhibit an obedient faith...
Should he fail in this endeavor by ongoing willful sin, or an ongoing departure from the exercise of faith and from the faith directed good works which God has ordained that he should walk in; or sustained slothfulness; or by any other means which would prevent him from finishing the set course laid our by God for him, he will fall from grace, being severed from Christ, having partaken of the grace of God in vain."
It is frightening how far from the truth one can fall. Pray God protect us all from this kind of apostasy.
The frightening thing is that this person was quoting John Owen approvingly elsewhere in his writing, but Owen himself wrote:
Q. In what doth the exercise of his priestly office for us chiefly consist?That's right, John Owen believed that those whom God regenerates, he will not fail to save. This is the message of John 6, this is the message of the Bible and this is the message which believers must cling to.
A. In offering up himself an acceptable sacrifice on the cross, so satisfying the justice of God for our sins, removing his curse from our persons, and bringing us unto him. — Chap. xiii.
I recommend the author read Owen on The Doctrine of the Saint's Perseverance freely available at Google Books.