The moment God hears, "It can't be done" from your mouth, it just may never be done. (Numbers 14:28)
Despite a fellow's unbelief, God's purpose will always be done, only that He will raise up another generation that will get the work done. (Numbers 13-14)
When Jesus said in John 14:14, "If ye shall ask anything in my name, I will do it," He was not joking. He didn't put any limit on what I can ask according to His will. (1 John 5:14-15)
Don't express doubt when it comes to God. Don't try to understand what He can do and how He will do it. Just believe
“Never say that anything is impossible; the moment someone says that something cannot be done, as far as he or she is concerned, it just may never be done.”
Memorise: And Caleb stilled the people before Moses, and said, Let us go up at once, and possess it; for we are well able to overcome it. (Numbers 13:30) 
