What is the ruling regarding one who doubts the power of Allah Almighty, and is he required to say anything after uttering the Two Testimonies of Faith?

Whoever falls into disbelief by denying or doubting the attributes of Allah must repent by pronouncing the Shahada (testimony of faith), affirming, and believing in what they denied or doubted. Disbelief does not befall a servant due to their ignorance or misinterpretation until the legal proof is established against them and the ambiguity is removed.