It must be remembered that Jesus delivered these truths in an age in which God's power was working many miracles through His servants in order to confirm the words being spoken (cf. Mark 16:20). The age of mortals performing miracles has since ceased (cf. I Cor. 13:8ff; Eph. 4:7ff). With those thoughts in mind, there is no reason why Jesus' words should not be interpreted literally and in harmony with the rest of the New Testament. If the cause of Christ could be furthered in no other way but for a mountain to literally be moved, then certainly such would have been accomplished through God's power. God could move a mountain through men of faith as easily as He could heal the sick and raise the dead through them.
"Therefore I say to you, whatever things you ask when you pray, believe that you receive them, and you will have them." Here is a general principle on prayer that stresses the importance of faith. If one prays while doubting, certainly the petition will not be granted (cf. James 1:6-8). Additionally, if one prays selfishly or for that which is unlawful, certainly his request will be denied (cf. James 4:2,3). Any prayer made to God must not be contrary to His will or it will not be granted (cf. I John 5:14,15; Luke 22:42).
The primary lesson for the disciples was that having miraculous ability alone was not enough. They must have true faith or they would fail to effectively use the power entrusted to them. The primary lesson for Christians today (since we lack the miraculous abilities of the first-century disciples) is that with strong faith and prayer whatever needs to be accomplished will be accomplished. Although we today may not be able to move any literal mountains, there is exceedingly great power in prayer that should be tapped by those filled with trust in God (cf. Eph. 3:20,21).
If one can always remember that he is the servant and God is the Master, he will find it easier to petition God appropriately and receive that which was requested.
"And whenever you stand praying, if you have anything against anyone, forgive him, that your Father in heaven may also forgive you your trespasses. But if you do not forgive, neither will your Father in heaven forgive your trespasses" (Mark 11:25,26; cf. Matt. 6:14).
It seems reasonable that Jesus here emphasizes the need to forgive because He has just performed a destructive miracle against a fig tree. He wants His disciples to understand that the miraculous gifts were not to be used with a vengeful, unforgiving spirit. When they suffered at the hands of the wicked, they were not to retaliate with miracles of condemnation.
One should be careful not to interpret this verse in a way that contradicts other passages regarding forgiveness. Jesus is not here teaching that one should automatically forgive any wrongs committed against him (cf. Luke 17:3,4). If an erring brother does not repent, one cannot forgive him. However, this is not to say that a grudge or evil disposition should be developed toward the brother who sinned (such is never appropriate). It is implied in this passage that the sin has been repented of and yet the individual praying has not yet forgiven. Refusing to forgive one who has repented makes one guilty of sin himself (cf. Matt. 6:15).