"without doubting" is a negative phrase that can be stated in the positive as "with complete certainty that God will answer." (See: [[rc://en/ta/man/translate/figs-litotes]])
This compares a person who doubts with the way water moves and keeps changing direction. AT: “is always moving back and forth between belief and unbelief” or “is always changing what he believes.” (See: [[rc://en/ta/man/translate/figs-simile]])
The word "doubleminded" refers to a person's thoughts when he is unable to make a decision. AT: "cannot decide if he will follow Jesus or not follow Jesus." (See: [[rc://en/ta/man/translate/figs-metonymy]])