The lack of evidence for the existence of its deity. Even proving that a deity exists is not enough; you would still need to prove that the deity you found is the one described by the Bible. And proving that the Christian deity exists would still not be enough; you would also need to prove that the Bible describes it accurately. And even then you would need to prove monotheism, i.e. that other possible gods aren’t real too.
The internal inconsistencies and factual errors in the Bible. Specialized websites like IronChariotsWiki and RationalWiki can give better descriptions of this problem than I could.
To me the weakest points of Christianism are two:
The lack of evidence for the existence of its deity. Even proving that a deity exists is not enough; you would still need to prove that the deity you found is the one described by the Bible. And proving that the Christian deity exists would still not be enough; you would also need to prove that the Bible describes it accurately. And even then you would need to prove monotheism, i.e. that other possible gods aren’t real too.
The internal inconsistencies and factual errors in the Bible. Specialized websites like IronChariotsWiki and RationalWiki can give better descriptions of this problem than I could.