Jesus said that hearing him and following him were distinguishing marks of his sheep. Commenting on John 10:27 -- "My sheep hear my voice, and I know them, and they follow me" -- Charles Spurgeon said that Christians "have a mark in their ear and a mark in their foot." Douglas MacMillan, who spent years tending sheep before he became a minister, explains that hearing the Good Shepherd's voice is more than an aural experience.
With every sheep that I brought home to take into the flock, the first thing I had to do was to take a big, long, sharp, killing knife. I was not going to cut their throats, but I was going to mark their ears. In Scotland we call it a 'lug mark'. It was my particular mark and it marked that sheep out as mine. Now that is not the kind of mark that Spurgeon meant when he said that the Christian has a mark in his ear. The kind of thing he was talking about was what Jesus had in mind here—'they hear my voice'. There are two words in Greek for hearing, and it is interesting to note that the one that is used here means not simply 'to hear a sound' but 'to hear and to understand'—hearing with understanding. (Gaelic has two words for hearing, as well, and makes exactly the same distinction.)
Now that is a perfect illustration of what happens when God's Spirit begins to work in the life of a sinner. They begin to hear. . . .
Often it's hard to distinguish the voice of Jesus from the other voices competing for our attention. How can one know that the Holy Spirit is at work calling us to repentance and faith? In short, when the gospel begins to have meaning and validity you are hearing the Shepherd's voice. When your thoughts and desires bend toward righteousness and holiness you are hearing the voice of Christ.
MacMillan tells how this happened in his own life. He grew up in a Christian home hearing the gospel from parents and pastors, but for 21 years he didn't have a clue what it was about. Then one day something changed.
The gospel was just jargon to me, and words like 'Come to Christ—trust in Christ—be born again' didn't mean a thing. Then all of a sudden the gospel began to have meaning for me. When a preacher said 'Come to Christ,' I knew exactly what he meant. When he said 'Trust your soul to the Lord Jesus,' I knew what he meant. When he said 'Christ died for your sins,' I thought that was wonderful. What was happening? I was not only hearing with the ear, I was understanding and I was listening, and I was drawing life into my soul. What had happened? I had become a sheep, and I understood the Shepherd's language and I knew the Shepherd's voice.
In theological terms what MacMillan experienced was regeneration, or the new birth. What was dead had been raised to life by the power of the Spirit. Spiritual blindness was replaced by spiritual sight. Of course, hearing the voice of Jesus necessarily leads to following him. More on that later.
Quotes from J. Douglas MacMillan, The Lord Our Shepherd (pp. 35-7)