In Mark 8:22-26 we read of Jesus healing a blind man. This healing seems to come in two phases, unlike most of Jesus other healings which were instant.
"And some people brought to him a blind man and begged him to touch him. And he took the blind man by the hand and led him out of the village, and when he had spit on his eyes and laid his hands on him, he asked him, “Do you see anything?” And he looked up and said, “I see men, but they look like trees, walking.” Then Jesus laid his hands on his eyes again; and he opened his eyes, his sight was restored..."
The obvious question to ask about this: 'Why was it not instant?'
Drawing attention to this David showed how some people are 'spiritually' healed instantly, yet others face a process & their 'spiritual healing is more gradual. This healing acts as a sort of illustration of that.
Personally i know some who have come to Christ in an instant, the Gospel has been so clear. Yet i also know others who've struggled to come to terms with & understand the Gospel over a period of time.
There are also many who find vices & sins that they had committed just vanish, yet others struggle with these sins for many years in their Christian life.