Is conversion sudden or gradual?  If by 'conversion' is really meant regeneration, the answer can only be 'sudden', for if words have meaning, 'birth' is a sudden and dramatic crisis.  Of course, there are months of preparation before birth.  Of course, too, there are years of growth after birth, but birth itself is an almost instantaneous experience.  So it is with the new birth.  There may be months in which the Holy Spirit begins to convince a man of his sin and turn his thoughts to Christ as the Saviour of sinners.  There may be months in which a man feels himself drawn by the magnetism of Christ.  There will also be years of development in the Christian life after the new birth.  'As newborn babes, desire the sincere milk of the Word, that ye may grow thereby' (1 Pet. 2:2).  The New Testament speaks of a growth in knowledge and holiness, in faith and love.  A Christian's progress is likened to the gradual development of a child into maturity.  But the months of pre-natal preparation and the years of post-natal growth must not be allowed to disguise the suddenness of birth itself.  Further, what growth is to birth, sanctification is to justification.  Justification, like birth, is sudden.  Sanctification, like growth, is gradual.  Justification is a legal metaphor and indicates the judge's sentence when he pronounces the sinner righteous.  The trial may take some time, and when it is over the justified sinner will take a lifetime to manifest in character the righteousness he has been accorded in standing, but the judge's sentence of justification is pronounced in a matter of seconds.  God's initial work in the soul then, whether we call it regeneration or justification, the experience of a new birth or the reception of a new status, is sudden.  It cannot be anything else.
-- John Stott