What did Jesus's family think about him when he first started his ministry?
that he was stoned on acid
Here the question is which forty days are you asking about. If it was the first 40 days , before Jesus started his earthly ministry . Or are you asking about the last 40 days when he was on the earth after he was on the cross. The first time was to prepare for his ministry. The last was to be with the disciples and leave tem with hope.
Christianity as it is know today was first upon the earth when Adam was taught the gospel. You seem to be implying that a date can be put to its first being referred to as Christianity. Perhaps to do so it might be acceptable then to say that it started either during Christs ministry or after the death of Christ.