God did not fail to convey his message. However, he gave humans free will, and he allows the devil to mislead. We have the ability to change his message and create different sets of beliefs because we fail to WANT to understand his message. What is easier, 1) to follow a message that says you have to continually internally and externally grow in holiness and grace with God’s sometimes silent help and repent of your sins, or 2) just to proclaim your devotion once and everything is good? So (this is but one example, but people either accept God’s message, or they find a path that, for them, is easiest to understand) God did not make a mistake, but we humans have taken that message and watered it down.
As for your proof, there is nothing in this world that we can know 100%. Mathematics (one of our most provable fields) is full of axioms, statements that are unprovable without relying on themselves. Physics has two big fields, Newtonian physics and Quantim mechanics. Each of those can be shown very definitively, but they practically disprove each other. You cannot know, 100% that I am a human and not a computer (though I rather hope I pass the Turing test). However, a man’s existence that is one of the most well supported in history (for the age in which he came) was Jesus. He was either God, as he said, lying, or delusional. Given the evidence surrounding his resurrection, delusional men do not have the consistency of thought that he displayed, and that no liar would die for his lies, most of us accept that he is God. He is proven to have started a Church that was to teach the Truth. Only one church can trace its roots (documented to have the same beliefs as the early Christians with what documents are existent and historically shown to follow one direct line) to him and him alone and that is the Catholic Church. The evidence is in favor, and that is all we can ask for on any principle. People believe in Anthropogenic Global Warming, micro/macro-evolution, the health benefits of vaccination, the health benefits of abortion, the cleanliness of manufacture of solar panlels, the need of mankind to commit mass suicide to save the earth, the economic justification of socialism/capitalism, the existence of Hogwarts, and more on less evidence than there is for Jesus and the Catholic Church. (I believe some, not all, of those are valid, but the point is each is believed by some group of people.) Proof is insufficient in every one of these cases to show, 100%, that such a thing is valid. If 100% proof is needed for us not to take something on blind faith, then we take our entire existence on blind faith.