Alma 30

 In answer to Korihor's claims, Alma answered - 

"...ye have the testimony of all these thy brethren, and also all the holy prophets? The scriptures are laid before thee, yea, and all things denote there is a God; yea, even the earth, and all things that are upon the face of it, yea, and its motion, yea, and also all the planets which move in their regular form do witness that there is a Supreme Creator."  (Alma 30:44)

And we have personal revelation - 

"...I say unto you, I know there is a God, and also that Christ shall come."  (Alma 30:39)