There are a group of people who claim to be "Messianic Jews." This is a group of people who are either natural born Jews who adopted Jesus as their messiah and other Christian practices or they are Christians who adopted Jewish practices alongside their belief in Jesus.
This question deals with what qualifies someone for a minyan.
If there are 9 Jews who are seeking a 10th for a minyan and the 10th happens to be a born Jew who converted into a Messianic Jewish lifestyle, is this Jew allowed to be counted as a 10th or does their belief in Jesus cancel out that action?
In this situation, the Jew was born a Jew (from a Jewish mother) and is considered halachically Jewish in this regard
In short: Can a person who as willingly entered into idolatry be considered a 10th for a minyan?
The problem with Messianic Judaism (it's not Judaism, just saying the name) is many of these people believe that they are following through on the Torah rather than rejecting it. They believe in Jesus but they also still follow Jewish laws in many respects, even Shabbos.
The question you're citing seems to indicate the person rejects Judaism entirely and even the belief in God and divinity.
– Michael Nov 29 '23 at 17:29