On Psalm 21

(Rev 1:5) ... Jesus Christ, who is the faithful witness, and the first begotten of the dead, and the prince of the kings of the earth. Unto him that loved us, and washed us from our sins in his own blood,
(Rev 1:6) And hath made us kings and priests unto God and his Father; to him be glory and dominion for ever and ever. Amen.

The word translated "prince" in Rev 1:5 means "ruler". Jesus rules over the kings and "hath made us kings and priests unto God and his Father". Christ "is Lord of lords and King of kings" (Rev 17:14). Various passages referring to kings could be applied to believers, since Christ "hath made us kings". However, this application is not possible in every passage. Ps 21 is a psalm that cannot be fully applied to believers, or in particular to king David. However, the psalm fits very well the Lord Jesus.

(Ps 21:1) The king shall joy in thy strength, O LORD; and in thy salvation how greatly shall he rejoice!
(Ps 21:2) Thou hast given him his heart's desire, and hast not withholden the request of his lips. Selah.

Before raising Lazarus from the dead, the Lord said "Father, I thank thee that thou hast heard me. And I knew that thou hearest me always: but because of the people which stand by I said it, that they may believe that thou hast sent me" (Jn 11:41-42). The statement that "Thou hast given him his heart's desire, and hast not withholden the request of his lips" (Ps 21:2) can be applied to Jesus, but not so easily to believers. After a believer is changed so that he perceives and desires what is right, then Ps 21:2 begins to be fulfilled also in his life. "Delight thyself also in the LORD; and he shall give thee the desires of thine heart" (Ps 37:4). Partial surrender to God is not sufficient. Rather, one should surrender in all his ways. "In all thy ways acknowledge him, and he shall direct thy paths" (Pr 3:6). The Lord Jesus was heard because of his reverence (Heb 5:7). His attitude was "not what I will, but what thou wilt" (Mk 14:36) even "unto death, even the death of the cross" (Php 2:8).

(Ps 21:3) For thou preventest him with the blessings of goodness: thou settest a crown of pure gold on his head.
(Ps 21:4) He asked life of thee, and thou gavest it him, even length of days for ever and ever.
(Ps 21:5) His glory is great in thy salvation: honour and majesty hast thou laid upon him.

(Jn 17:4)EMTV I have glorified You on the earth. I have finished the work which You have given Me to do.
(Jn 17:5) And now, O Father, glorify Me alongside Yourself, with the glory which I had with You before the world was.

(Ps 21:6) For thou hast made him most blessed for ever: thou hast made him exceeding glad with thy countenance.

The words for "made him most blessed" could be translated "set him to be blessings". Anyone who has Christ has blessings: "God ... hath blessed us with all spiritual blessings in heavenly places in Christ" (Eph 1:3).

(Ps 21:7) For the king trusteth in the LORD, and through the mercy of the most High he shall not be moved.

For Christ, "to be moved" probably meant "to be kept from doing all the work". Jesus eagerly desired to do the entire will of the Father, so he prayed much. He "offered up prayers and supplications with strong crying and tears" (Heb 5:7). We could guess that he prayed much so that he might be able to do all the work to which he was called. "My food is to do the will of Him who sent Me, and to finish His work" (Jn 4:34)EMTV.

(Ps 21:8) Thine hand shall find out all thine enemies: thy right hand shall find out those that hate thee.
(Ps 21:9) Thou shalt make them as a fiery oven in the time of thine anger: the LORD shall swallow them up in his wrath, and the fire shall devour them.
(Ps 21:10) Their fruit shalt thou destroy from the earth, and their seed from among the children of men.

(Mt 13:41) The Son of man shall send forth his angels, and they shall gather out of his kingdom all things that offend, and them which do iniquity;
(Mt 13:42) And shall cast them into a furnace of fire: there shall be wailing and gnashing of teeth.

(Ps 21:11) For they intended evil against thee: they imagined a mischievous device, which they are not able to perform.
(Ps 21:12) Therefore shalt thou make them turn their back, when thou shalt make ready thine arrows upon thy strings against the face of them.
(Ps 21:13) Be thou exalted, LORD, in thine own strength: so will we sing and praise thy power.

In Ps 21:13, "we" could be understood as referring to Christ and his people (Heb 2:12).