In your opinion what causes christians to believe in Jesus

[Thumpalumpacus]

And when we observe that the grass is dry, what does that tell us about the proposition that it rained?

That it's disconfirmed.

Wrong. It may be that after the rain the sun came out again and dried it up (I live in Texas, sunshowers are a regular event), or it may mean that you saw rain falling but that the Earth's radiant heat evaporated the water quickly.