Doubt is a feeling of uncertainty about the truth, reality or distrust of anything. In spiritualism doubt is just opposite of the Faith. Faith in God as the Supreme reality is the essence of any religion.
If you do not have faith in God then you cannot believe in the words of God. Such doubting person cannot find peace in life & cannot attain to Divine consciousness. Doubting soul finds happiness neither in this world nor in the next, while the faithful one's attain peace & reach to God easily. Below verse of Bhagwat Gita explains that only.
"ajnas chasraddadhanas cha
nayam loko ’sti na paro
na sukham samsayatmanah" (Bhagwat Gita: Chapter Four verse 40)
"Sri Krishna said: But ignorant and faithless human beings who doubt the revealed scriptures do not attain God consciousness; they fall down. For the doubting soul there is happiness neither in this world nor in the next."
Jai Sri Krishna