Jesus answered and said unto them, Truly I Say unto you, If you have Faith, and Doubt not, you shall not only do this which is done to the fig Tree, but also if you shall Say unto this mountain, Be you removed, and be you cast into the sea; it shall be done. - Mat 21:21
For truly I Say unto you, That whoever shall Say unto this mountain, Be you removed, and be you cast into the sea; and shall not Doubt in his Heart, but shall Believe that those things which he says shall come to pass; he shall have whatsoever he says. - Mar 11:23
And Seek not you what you shall eat, or what you shall drink, neither be you of doubtful Mind. - Luk 12:29
Arise therefore, and get you down, and go with them, doubting nothing: for I have sent them. - Act 10:20
When you are Doubtless about something you are 100% confident in it. You are assured.
Doubt is Unbelief. Doubt is imperfect Faith.
Any time a person attempts to do something that is challenging Physically, Mentally or Spiritually they need to be Doubtless.
Any particle of Doubt will cause that person to falter in their attempt. It will result in failure.
Be Doubtless or as Jesus said: "Have Faith in God!".