He says to me, “Put away doubting from you and do not hesitate to ask of the Lord, saying to yourself, 'How can I ask of the Lord and receive from Him, seeing I have sinned so much against Him? 'Do not thus reason with yourself, but with all your heart turn to the Lord and ask of Him without doubting, and you will know the multitude of His tender mercies; that He will never leave you, but fulfil the request of your soul. For He is not like men, who remember evils done against them; but He Himself remembers not evils, and has compassion on His own creature. Cleanse, therefore, your heart from all the vanities of this world, and from the words already mentioned, and ask of the Lord and you will receive all, and in none of your requests will you be denied which you make to the Lord without doubting. But if you doubt in your heart, you will receive none of your requests. For those who doubt regarding God are double-souled, and obtain not one of their requests. But those who are perfect in faith ask everything, trusting in the Lord; and they obtain, because they ask nothing doubting, and not being double-souled. For every double-souled man, even if he repent, will with difficulty be saved. Cleanse your heart, therefore, from all doubt, and put on faith, because it is strong, and trust God that you will obtain from Him all that you ask. And if at any time, after you have asked of the Lord, you are slower in obtaining your request [than you expected], do not doubt because you have not soon obtained the request of your soul; for invariably it is on account of some temptation or some sin of which you are ignorant that you are slower in obtaining your request. Wherefore do not cease to make the request of your soul, and you will obtain it. But if you grow weary and waver in your request, blame yourself, and not Him who does not give to you. Consider this doubting state of mind, for it is wicked and senseless, and turns many away entirely from the faith, even though they be very strong. For this doubting is the daughter of the devil, and acts exceedingly wickedly to the servants of God. Despise, then, doubting, and gain the mastery over it in everything; clothing yourself with faith, which is strong and powerful. For faith promises all things, perfects all things; but doubt having no thorough faith in itself, fails in every work which it undertakes. You see, then,” says he, “that, faith is from above— from the Lord — and has great power; but doubt is an earthly spirit, coming from the devil, and has no power. Serve, then, that which has power, namely faith, and keep away from doubt, which has no power, and you will live to God. And all will live to God whose minds have been set on these things.”
Of Grief, and Not Grieving the Spirit of God Which is in Us.
Source: The Pastor (or "The Shepherd") (New Advent)