Paul Helm in The Beginnings: Word & Spirit in Conversion, pp 9, 22-23:
[T]here are many different kinds of consciously-felt experiences of God’s grace, perhaps as many different kinds as there are different people who experience that grace. At one extreme there are those whose awareness of becoming a Christian is slow and almost imperceptible. God has brought them gradually to faith in Christ and to full commitment to Him. There are others, like Paul, or Augustine, whose conversion was a sudden crisis, a distinct, datable event.
During an intense experience of contrition for his sins, Augustine [I’ve used the Henry Chadwick translation below]
suddenly I heard a voice from the nearby house chanting as if it might be a boy or a girl (I do not know which), saying and repeating over and over again ‘Pick up and read, pick up and read.’ At once my countenance changed, and I began to think intently whether there might be some sort of children’s game in which such a chant is used. But I could not remember having heard of one. I checked the flood of tears and stood up. I interpreted it solely as a divine command to me to open the book and read the first chapter I might find. For I had heard how Antony happened to be present at the gospel reading, and took it as an admonition addressed to himself when the words were read: ‘Go, sell all you have, give to the poor, and you shall have treasure in heaven; and come, follow me’ (Matt. 19: 21). By such an inspired utterance he was immediately ‘converted to you’ (Ps. 50: 15). So I hurried back to the place where Alypius was sitting. There I had put down the book of the apostle when I got up. I seized it, opened it and in silence read the first passage on which my eyes lit: ‘Not in riots and drunken parties, not in eroticism and indecencies, not in strife and rivalry, but put on the Lord Jesus Christ and make no provision for the flesh in its lusts’ (Rom. 13: 13–14). I neither wished nor needed to read further. At once, with the last words of this sentence, it was as if a light of relief from all anxiety flooded into my heart. All the shadows of doubt were dispelled.
In his booklet, A Self-Portrait, the Anglican leader of the last century, J.C. Ryle, remarked that:
The circumstances which led to a complete change in my character were very many and very various, and I think it right to mention them. It was not a sudden immediate change but very gradual. I cannot trace it to any one person, or any one event or thing, but to a singular variety of persons and things. In all of them I believe now the Holy Ghost was working, though I did not know it at the time.
Such variety of experience, recorded throughout the history of the church, is to be found already in Scripture. The experience of Zacchaeus (Luke 19:1-10) is different from that of Saul of Tarsus (Acts 9:12-22), that of Lydia (Acts 16:14) is different from the experience of Nathanael (John 1:45-51).