A person who does not know if God exists. They are skeptical but do not deny the possibility, whereas an atheist categorically denies the
existence of God.
The existence of God cannot be definitely proven or disproven and we must accept by faith that God exists. "Without faith it is impossible to please God, because anyone who comes to Him must believe that He exists and that He rewards those who earnestly seek Him" (Heb 11:6). God is spirit and so cannot be seen or touched (Jn 4:24). However, the existence of God is readily observed in the universe, confirmed in the hearts of believers and sensed in what He has made (Ps 19:1-4; Eccl 3:11; Rom 1:18-22). Many people doubt God exists because they do not understand or agree with the things He does or permits. Yet in our finite minds we can't comprehend the wisdom and power of an infinite God. He gets blamed for the problems in the world, when in fact it was humans who sinned in the Garden of Eden and we are reaping some of the consequences.
See also: atheist, existence of God, faith, God.