In Luke 9.37-45 and Mark 9.14-31 the same story is recorded: a man comes to Jesus asking him to help heal his son. The gospel of Mark records him saying to Jesus "if you can do anything." Jesus calls him out for his lack of belief and the father replies, "I believe, help me overcome my unbelief."
Alastair Sterne preaches from these passages and identifies this father's statement as the crux of sincere faith. We can turn to God with the smallest bit of faith and God will not disappoint us.