In this passage, Christ said: "If ye have faith as a grain of mustard seed, ye shall say unto this mountain, Remove hence to yonder place; and it shall remove; and nothing shall be impossible unto you."
Christ was not suggesting that we could simply perform tricks to amaze our friends, but that if we have the faith for some need, and asked for it in faith, our prayer will be answered. But faith is itself a gift from God (1 Corinthians 12:9; Ephesians 2:8), and God will only give us the faith to ask for things that are in accordance with His will. St. John the Apostles says this explicitly in 1 John 5:14-15: "And this is the confidence that we have in Him, that, if we ask any thing according to His will, He heareth us: and if we know that He hear us, whatsoever we ask, we know that we have the petitions that we desired of Him."
Commenting on this, St. Justin Popovich says:
"True human confidence is in the confidence of God, i.e., when man's confidence is completely in the confidence of God, then it is infinite, salvific, and almighty. Its boundaries are: Nevertheless not as I will, but as Thou wilt (Matt. 26:39). What is the will of God towards us, the human race? To achieve, through faith in the Son of God, eternal life. The will of God is completely and perfectly expressed in the God-man Christ, through His Gospel, through His works of salvation (cf. Eph. 1:9-10). Therefore, our confidence is in that we ask for that which belongs to Christ and is evangelical, and -- we receive it from God. It would not be confidence but insolence, shameless, and blind slavery to sin to ask from God something that is unevangelical and to wait for Him to fulfill it for us. Ultimately, that would mean wanting God to be a co-participant in our sin, to bring Him into our love of sin. The reason why our all-good and omniscient God does not fulfill many of our prayers is because they are not according to His will. The stipulation for our prayers to be fulfilled is that they be according to the will of God, and you shall receive. That which is according to the will of God is what is of the Gospel and in accordance with the Gospel. Through this, man's confidence is vouchsafed movement and life within God's infinite Goodness, God''s infiniteness. This is most perfect and the sweetest confidence. Everything else, directly or indirectly, is slavery to sin, death, the devil, and evil" (Archimandrite Justin Popovich, Commentary on the Epistles of St. John the Theologian, trans. Radomir M. Plavsic (Alhambra, CA: Sebastian Press, 2009) p.76f).
There is nothing that I can add to these words of St. Justin.
