A slave and a slave trader both had chains.

John Newton wrote a number of verses to "Amazing Grace."  Three verses Chris Tomlin uses in his song, "Amazing Grace" (My Chains are Gone), are:

Amazing grace how sweet the sound

That saved a wretch like me

I once was lost, but now I'm found

Was blind, but now I see

'Twas grace that taught my heart to fear

And grace my fears relieved

How precious did that grace appear

The hour I first believed

The Lord has promised good to me

His word my hope secures 

He will my shield and portion be

As long as life endures

Two other verses John Newton wrote for "Amazing Grace" were:

Yea, when this flesh and heart shall fail, 

And mortal life shall cease:

I shall possess, within the veil,

A life of joy and peace

This earth shall soon dissolve like snow,

The sun forbear to shine:

But God, Who call's me here below,

Will be for ever mine.

Chris Tomlin, a contemporary song writer, saw the fifth verse, and included it in this song. 

John Newton had a believing mother, but she died when he was seven.  At the age of eleven, John left school to begin life as a rough, debauched seaman.  He helped transport slaves to America.  

One day, John was alarmed after a fierce storm at sea, and began to read, "The imitation of Christ" by Thomas a Kemps.  His conversion followed, and his life changed.  John joined the fight against slavery in 1788, and was a friend of William Wilberforce in England.

For the last 43 years of his life, John Newton preached the gospel in Olney and London.  At the age of 82, Newton said, "My memory is nearly gone, but I remember two things--that I am a great sinner, and that Christ is a great Savior."

Chis Tomlin added a chorus to his song:

My chains are gone, I've been set free.

My God, My Savior has ransomed me.

And like a flood his mercy reigns.

Un-ending love, amazing grace.