"The 'grace' that made him (Paul) a preacher drove him across continents and across seas; it made him preach day and night with tears and pleading, it was the most vital force in his life. This was the thing that "constrained" him and made him say; "Woe is me if I preach not the gospel". He was driven by thought of these riches of God's grace and the ignorance of men and women concerning them. It was his chief reason for writing this Epistle to the Ephesians".
Dr Martyn Lloyd-Jones - "Ephesians; God's Ultimate Purpose (Ephesians 1)" - Banner of Truth Trust, Edinburgh (1978) - (p174).