People that have been christened are therefore considered "affiliates forever" (pun intended) by the catholic church.
In every of their listings and statistics, you are considered as an active member of their church.
The only way to disappear from their church is to apostatize, one thing that roman catholic church doesn't like at all.

Apostate, literally designates anyone who changes his religion, whatever may be the motive; but, by custom, the word is always used in an injurious sense, as equivalent to one who, in changing his creed, is actuated by unworthy motives.

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Entry from Everybody's Cyclopedia, 1912.

A*pos"tate (#), n. [L. apostata, Gr. , fr. . See Apostasy.]

1.

One who has forsaken the faith, principles, or party, to which he before adhered; esp., one who has forsaken his religion for another; a pervert; a renegade.

2. R. C. Ch.

One who, after having received sacred orders, renounces his clerical profession.

 

© Webster 1913.

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A*pos"tate, a.

Pertaining to, or characterized by, apostasy; faithless to moral allegiance; renegade.

So spake the apostate angel. Milton.

A wretched and apostate state. Steele.

 

© Webster 1913.

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A*pos"tate, v. i. [L. apostatare.]

To apostatize.

[Obs.]

We are not of them which apostate from Christ. Bp. Hall.

 

© Webster 1913.