Apostasy

None sink so far into hell as those that come nearest heaven, because they fall from the greatest height.
William Gurnall

[Apostasy is] the deliberate repudiation and abandonment of the faith that one has professed (Heb. 3:12). Apostasy differs in degree from heresy. The heretic denies some aspects of the Christian faith but retains the Christian name. Again, the transfer of membership from one denomination to another of the same faith is not apostasy. It is also possible for a person to deny the faith, as Peter did, then reaffirm it at a later time.
L.G. Whitlock Jr. from Evangelical Dictionary of Theology (2nd Ed.)