Definition: The claim you can tell a believer he can never lose his salvation based on his initial demonstration of faith in Christ, no matter how wickedly he continuously sins in the future. Different Definition
Problem 1: Many verses explicitly refute Once Saved Always Saved (OSAS).
Problem 2: Many verses say those who continue to behave unrighteously (lying, adultery, etc.) are lost. We cannot predict that a new believer will not fall away into damnable sin. Many do.
Problem 3: Belief in OSAS makes a person more likely to sin, since he believes he can willfully disobey God and God will accept him anyway.
Summary: The many verses, such as Ephesians 2:8-9, used to "prove" salvation cannot be lost are being taken out of context. In context they refer to repentant people who are characterized by holy living.