The Shepherd of Hermas

Therefore purify your heart from double-mindedness, but put on faith, because it is mighty, and believe God, that you shall obtain all your requests which you make. And if ever you make any petition from the Lord, and receive it but slowly, do not be double-minded because you have not received the request of your soul speedily, for in every case it is because of some temptation or some transgression, of which you are ignorant, that you receive your request slowly.

Do not therefore cease from making the request of your soul, and you shall receive it. But if you grow weary, and are double-minded in your request, blame yourself and not him who gives to you.

Consider this double-mindedness; for it is wicked and foolish, and uproots many from the faith, yes, even those who are very faithful and strong. For this double-mindedness

Despise therefore double-mindedness, and master it in every act, putting on the faith which is strong and powerful. For faith promises all things, perfects all things. But the double-mindedness which has no full faith in itself fails in all deeds which it undertakes.

You see, then, said he, that faith is from above, from the Lord, and has great power; but double-mindedness is an earthly spirit, from the devil, and has no power.

Do you, therefore, serve the faith which has power, and refrain from the double-mindedness which has no power, and you shall live to God, and all who have this mind shall live to God.