Doubting is always put under the ban, because it stands as a foe to faith and hinders effectual praying. In the First Epistle to Timothy Paul gives us an invaluable truth relative to the conditions of successful praying, which he thus lays down: "I will therefore that men pray everywhere, lifting up holy hands, without wrath and doubting."

All questioning must be watched against and eschewed. Fear and peradventure have no place in true praying. Faith must assert itself and bid these foes to prayer depart.

Too much authority cannot be attributed to faith; but prayer is the sceptre by which it signalizes its power. How much of spiritual wisdom there is in the following advice written by an eminent old divine.
"Would you be freed from the bondage to corruption?" he asks. "Would you grow in grace in general and grow in grace in particular? If you would, your way is plain. Ask of God more faith. Beg of Him morning, and noon and night, while you walk by the way, while you sit in the house, when you lie down and when you rise up; beg of Him simply to impress Divine things more deeply on your heart, to give you more and more of the substance of things hoped for and of the evidence of things not seen."

Great incentives to pray are furnished in Holy Scriptures, and our Lord closes His teaching about prayer, with the assurance and promise of heaven. The presence of Jesus Christ in heaven, the preparation for His saints which He is making there, and the assurance that He will come again to receive them how all this helps the weariness of praying, strengthens its conflicts, sweetens its arduous toil! These things are the star of hope to prayer, the wiping away of its tears, the putting of the odour of heaven into the bitterness of its cry. The spirit of a pilgrim greatly facilitates praying. An earth-bound, earth-satisfied spirit cannot pray. In such a heart, the flame of spiritual desire is either gone out or smouldering in faintest glow. The wings of its faith are clipped, its eyes are filmed, its tongue silenced. But they, who in unswerving faith and unceasing prayer, wait continually upon the Lord, do renew their strength, do mount up with wings as eagles, do run, and are not weary, do walk, and not faint.

Mark 11:23-26

23 Truly I tell you, whoever says to this mountain, Be lifted up and thrown into the sea! and does not doubt at all in his heart but believes that what he says will take place, it will be done for him.

24 For this reason I am telling you, whatever you ask for in prayer, believe (trust and be confident) that it is granted to you, and you will [get it].

25 And whenever you stand praying, if you have anything against anyone, forgive him and let it drop (leave it, let it go), in order that your Father Who is in heaven may also forgive you your [own] failings and shortcomings and let them drop.

26 But if you do not forgive, neither will your Father in heaven forgive your failings and shortcomings.
AMP