John Calvin Commentary Luke 8:38

"But the man from whom the demons were gone out prayed him that he might be with him: but he sent him away, saying," — Luke 8:38 (ASV)

And the men requested. The Gadarenes cannot endure to have Christ among them, but the man who had been delivered from the devil desires to leave his own country and follow Him. From this we learn how wide the difference is between the knowledge of the goodness of God and the knowledge of the power of God. Power strikes people with terror, makes them flee from the presence of God, and drives them to a distance from Him; but goodness draws them gently and makes them feel that nothing is more desirable than to be united to God.

We cannot determine with certainty why Christ refused to have this man as one of His followers, unless it was that He expected the man to make himself more extensively useful by communicating to his Gentile countrymen the remarkable and extraordinary act of kindness he had received. This, indeed, the man actually did, as Mark and Luke assure us.