In life, we almost never wait for 100% guarantees. We trust that the dentist is a dentist, the taxi driver is a taxi driver, and so on --and put our lives in their hands --on flimsy tacit evidence. Yet, when it comes to a simple good deed, people demand 100% proof that this is really what G-d wants them to do!
And if you shall say: What shall we eat in the seventh year. if we do not plant, nor gather our crops? But I will command my blessing upon you in the sixth year, and it shall bring forth fruit for three years...