"There is a Fountain Filled with Blood" is a well-known hymn written by William Cowper (1731-1800).  The imagery of a fountain is found in Zechariah 13:1, "In that day shall be a fountain opened to the house of David and to the inhabitants of Jerusalem for sin and uncleanness."

The first verse says:

There is a fountain filled with blood

Drawn from Immanuel's veins;

And sinners, plunged beneath that flood,

Lose all their guilty stains.

"Lose all their guilty stains."  A definition of guilt can be self-consciousness of sin and failure.  Guilt can cause us to hide.  But the next verse tells of the dying thief on the cross who, instead, turned to Jesus:

The dying thief rejoiced to see

That fountain in his day;

and there may I, tho' vile as he,

Wash all my sins away.

The thief said, "We deserve to die for our evil deeds, but this man hasn't done anything wrong."  Then the thief said, "Jesus, remember me when you come into your kingdom."  (Luke 23:41-43)  And Jesus replied, "I assure you, today you will be with Me in paradise."

William Cowper's pastor was John Newton, the writer of "Amazing Grace."  Together, they put together a hymnal with 349 hymns, 67 of them written by Cowper.  In verse 5, Cowper says the hymns he sang on earth won't compare to the songs he will sing in heaven.

When this poor lisping stamm'ring tongue

Lies silent in the grave,

Then in a nobler, sweeter song

I'll sing Thy pow'r to save.

I'll sing Thy pow'r to save,

I'll sing Thy pow'r to save.

Then in a nobler, sweeter song

I'll sing Thy pow'r to save.

The other two verses are:

Dear dying Lamb, Thy precious blood 

Shall never lose its pow'r

Till all the ransomed Church of God

Be saved, to sin no more.

E'er since by faith, I saw the stream

Thy flowing wounds supply,

Redeeming love has been my theme

And shall be till I die.