The belief that salvation cannot be lost is known as "eternal security" and "once saved always saved". However, whilst we are saved by believing in Jesus (John 3:16), in order to keep our salvation, we must keep on believing in Jesus. This is evident from the use of the words "if", “firm to the end” and "continue" in the following verses (I have capitalised to draw attention to them):
Dr. Stuart Pattico
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Verses marked NIV are from THE HOLY BIBLE, NEW INTERNATIONAL VERSION®, NIV® Copyright © 1973, 1978, 1984, 2011 by Biblica, Inc.® Used by permission. All rights reserved worldwide. Verses marked HCSB are taken from the Holman Christian Standard Bible®, Copyright © 1999, 2000, 2002, 2003, 2009 by Holman Bible Publishers. Used by permission. Holman Christian Standard Bible®, Holman CSB®, and HCSB® are federally registered trademarks of Holman Bible Publishers. Verses marked NKJV are taken from the New King James Version®. Copyright © 1982 by Thomas Nelson. Used by permission. All rights reserved. Verses marked NLT are from Holy Bible, New Living Translation, copyright © 1996, 2004, 2015 by Tyndale House Foundation. Used by permission of Tyndale House Publishers Inc., Carol Stream, Illinois 60188. All rights reserved. Quotations marked NEB are from The New English Bible: The New Testament © ‘The
Delegates of the Oxford University Press and the Syndics of the Cambridge University Press 1961, 1970.