Once Saved Always Saved?

QUESTION: A class of people here teach that a person once saved cannot be lost. Their chief proof text is John 10:28, "I give unto them eternal life and they shall never perish, neither shall any pluck them out of my hand." Please explain.

ANSWER: All of God's promises of spiritual blessings are conditioned expressly or by implication. The implied condition here is strongly hinted to the Greek reader in the context by the use of the present tense, denoting continuance: "My sheep are hearing my voice and they are following me." Such persevering believers have eternal life. Says Bishop Westcott: 

"If any man falls in his spiritual life, it is not from want of divine grace, nor from the overwhelming power of adversaries, but from his neglect to use that which he may or may not use. We cannot be protected against ourselves in spite of ourselves. The difficulty in this case is only one form of the difficulty involved in the relation of an infinite to a finite being. The sense of the divine protection is at any moment sufficient to inspire confidence, but not to render effort unnecessary." 

So long as obedient faith continues, the spiritual life continues, but when faith lapses, the life, which might have been everlasting, also lapses. This is impressively taught in the parable of the vine in John 15:1-7. Fruitless branches of the true vine are burned. There is no other rational exegesis.

— Steele's Answers pp. 100, 101.