It’s a foundation of Christianity. See I Corinthians 15:3,4.
It may be the foundation of Christianity and it may also have been true.
The trouble is that there is NO documentation of those facts until many decades after the event and under two circumstances -
1. By Paul who never was known to have met anyone by the the name of Jesus or Jesua.
2. By the disciples who had not written ANYTHING for well over two full decades and then only at the urging of Paul.
3. The writings of the disciples was “probably” inscribed (by a single scribe) whos writings were strangely similar in most parts (or many party at least) which indicates a singular writer who elaborated of vague verbal recitation by the disciples.
Other “Roman” documentation mentions nothing about Jesus and/or his disciples by name but does mention that there were many “Self Proclaimed” Messiahs. John the Baptist was mentioned many times but considered harmless so they left him alone as he was apparently doing no harm and was actually helping keep the population calm and non violent.