Never give up praying, not even though Satan should suggest to you that it is in vain for you to cry unto God. Pray in his teeth; "pray without ceasing." If for awhile the heavens are as brass and your prayer only echoes in thunder above your head, pray on; if month after month your prayer appears to have miscarried, and no reply has been vouchsafed to you, yet still continue to draw nigh unto the Lord. Do not abandon the mercy-seat for any reason whatever. If it be a good thing that you have been asking for, and you are sure it is according to the divine will, if the vision tarry wait for it, pray, weep, entreat, wrestle, agonise till you get that which you are praying for. If your heart be cold in prayer, do not restrain prayer until your heart warms, but pray your soul unto heat by the help of the everblessed Spirit who helpeth our infirmities. If the iron be hot then hammer it, and if it be cold hammer it till you heat it.
Never cease prayer for any sort of reason or argument. If the philosopher should tell you that every event is fixed, and, therefore, prayer cannot possibly change anything, and, consequently, must be folly; still, if you cannot answer him and are somewhat puzzled, go on with your supplications notwithstanding all. No difficult problem concerning digestion would prevent your eating, for the result justifies the practice, and so no quibble should make us cease prayer, for the assured success of it commends it to us. You know what your God has told you, and if you cannot reply to every difficulty which man can suggest, resolve to be obedient to the divine will, and still "Pray without ceasing." Never, never, never renounce the habit of prayer, or your confidence in its power.
From a sermon by Charles Haddon Spurgeon entitled "Pray Without Ceasing," delivered March 10, 1872. Image by Lali Masriera under Creative Commons License.
This is so true. NEVER GIVE UP. I prayed for 20 years for my dad to be saved, and just a few months before he passed away he trusted Christ as his savior.