You would make me believe the moon is made of green cheese.

The speaker rejects a claim as so implausible that accepting it would seem as foolish as believing the moon is made of cheese.

The saying also challenges the person making the claim: the speaker feels they are being asked to accept something plainly unbelievable.

Use this as a skeptical reply when someone presents a claim you consider wildly implausible. It is pointed and rhetorical, so it may sound more confrontational than a simple request for evidence.