I've found the pearl of greatest price
A Song of Praise for Christ
Mason's text in this hymn is from the parable in Matthew 13:45, 'Again, the kingdom of heaven is like unto a merchant man, seeking goodly pearls: Who, when he had found one pearl of great price, went and sold all that he 
had, and bought it.'

The five verses that follow are the form in which the hymn is now sung; it can be seen from the original at the bottom of the page that it has been heavily edited, especially the fourth 'modern' verse.

I've found the pearl of greatest price 
    My heart doth sing for joy;
And sing I must, for Christ is mine;
    Christ shall my song employ.

Christ is my Prophet, Priest and King; 
    A Prophet full of light, 
My great High Priest before the throne, 
    My King of heav’nly might.

Mason is drawing on the Book of 
Malachi in verse three:

For He indeed is Lord of lords, 
    And He the King of kings; 
He is the Sun of righteousness, 
    With healing in His wings.

Christ is my Peace; He died for me, 
    For me He gave His blood; 
And as my wondrous Sacrifice, 
    Offered Himself to God.

Christ Jesus is my All in all, 
    My Comfort and my Love, 
My Life below, and He shall be 
    My Joy and Crown above.

Mason's original words show
 I’ve found the Pearl of greatest Price, 
    My Heart doth sing for Joy; 
And sing I must; a Christ I have; 
    O what a Christ have I? 
Christ is the Way, the Truth and Life, 
    The Way to God and Glory, 
Life to the Dead, the Truth of Types, 
    The Truth of Ancient Story.

Christ is a Prophet, Priest and King; 
    A Prophet full of Light, 
A Priest that stands 'twixt God and Man, 
    A King that Rules with Might. 
Christ's Manhood is a Temple, where, 
    The Altar, God doth Rest: 
My Christ, he is the Sacrifice; 
    My Christ, he is the Priest.
Sources and derivatives

'Come my Way, my Truth, my Life' by George Herbert (1593-1632) is said to be an inspiration for Mason's hymn, though the connection is mostly in the unused second half of Mason's original first verse:

Come, my Way, my Truth, my Life:  
    such a way as gives us breath,  
    such a truth as ends all strife,  
    such a life as killeth death.

Come, my Light, my Feast, my Strength:  
    such a light as shows a feast,  
    such a feast as mends in length,  
    such a strength as makes his guest.

Come, my Joy, my Love, my Heart:  
    such a joy as none can move,  
    such a love as none can part,  
    such a heart as joys in love.

John Newton (1725-1807) draws on Mason's second verse in 'How sweet the name of Jesus sounds':

Jesus! my Shepherd, Brother, Friend, 
    My Prophet, Priest and King; 
My Lord, my Life, my Way, my End, 
    Accept the praise I bring.

Charles Wesley (1707-1788) echoes Mason's  
third verse in 'Hark the herald angels sing':

Hail, the heaven-born Prince of Peace! 
    Hail, the Sun of Righteousness! 
Light and life to all he brings, 
    Risen with healing in His wings. 

how much this hymn has changed
My Christ, he is the Lord of Lords, 
    He is the King of Kings; 
He is the Son of Righteousness, 
    With Healing in his Wings. 
My Christ, he is the Tree of Life 
    Which in God's Garden grows, 
Whose Fruit does feed, whose Leaves do heal; 
    My Christ is Sharon's Rose.

Christ is my Meat, Christ is my Drink, 
    My Physick and my Health, 
My Peace, my Strength, my Joy, my Crown, 
    My Glory, and my Wealth. 
Christ is my Father and my Friend, 
    My Brother and my Love, 
My Head, my Hope, my Counsellor, 
    My Advocate above.

My Christ, he is the Heaven of Heaven; 
    My Christ what shall I call? 
My Christ is First, my Christ is Last, 
    My Christ is All in All.