I am looking at a birth record from Alsace, 1751. In the margin is written the word "apostatavit". I have not been able to figure out what that means.
Apostatavit is a Latin word meaning (roughly -- my Latin is a little rusty after 44 years) 'has forsaken his religion' -- i.e. it denotes an apostate. See Wikipedia for more on apostasy
As this was an annotation added later to a church record, I would interpret it as saying that the individual whose birth was recorded had themselves later become an apostate and was unlikely to appear later in the register. Other entries were annotated with the date the individuals died/obiit -- a pity this annotation was not dated.
When looking for marriage, birth of children and death, you should check both the church/faith which recorded the birth (as you can't know from this record when they apostatised) and other denominations. Civil registration in Alsace as far as I can discover began in 1792 (it belonged to France at the time) so you might find the registrations of a late marriage, birth of children and death in the civil records.