Your choice...

But I think the song could be viewed as his start of a "new life" and that his personal changes came about more slowly. From Wikipedia:

John Newton has been criticised by some modern writers for continuing to participate in the slave trade while, at the same time, holding to strong Christian convictions. This has often been characterised as hypocrisy. But this should be seen in the light of his own estimation of the true timing of his Christian conversion. Writing about the definite change that had happened to him in 1748, Newton would later write:

'I was greatly deficient in many respects...I cannot consider myself to have been a believer (in the full sense of the word) till a considerable time afterwards."