Here He wants to teach us what in all Scripture is considered the chief thing in prayer: the assurance that prayer will be heard and answered. Observe how He uses words which mean almost the same thing, and each time repeats the promise so distinctly: ‘Ye shall receive, ye shall find, it shall be opened unto you;’ and then gives as ground for such assurance the law of the kingdom: ‘He that asketh, receiveth; he that seeketh, findeth; to him that knocketh, it shall be opened.’ We cannot but feel how in this sixfold repetition He wants to impress deep on our minds this one truth, that we may and must most confidently expect an answer to our prayer.
Andrew Murray