Suppose when the father, in receiving back the prodigal, had said, “Go into the house, and get the best robe and put it on and come to me,” there might have been some meaning in the son’s saying, “I can’t!” But when the father says to the servants, “Bring forth the best robe and put it on him,” such an excuse would have been absurd, and would only have betrayed the son’s unwillingness to receive the robe at all. For the father leaves nothing for the son to do; all he desires is that he should receive: and it is as if he had said, “Allow me to clothe you; allow me to put the best robe upon you.” He undertakes for everything; for the putting on the robe as well as for the robe itself.
That which many call the difficulty of believing is the essence of self-righteousness. Yes; it is this that lies at the root of, or rather is the root of, this difficulty. Men cling to self as the lad clung to the rope; they will not let it go; and they cry all the while that they can’t.

“I Can’t Let Go”, Hoartius Bonar in How Shall I Go to God

Bonar alludes to the parable of the Prodigal Son. It appears in Luke 15 and reads as follows:

11 And he said, “There was a man who had two sons.
12 And the younger of them said to his father, ‘Father, give me the share of property that is coming to me.’ And he divided his property between them.
13 Not many days later, the younger son gathered all he had and took a journey into a far country, and there he squandered his property in reckless living.
14 And when he had spent everything, a severe famine arose in that country, and he began to be in need.
15 So he went and hired himself out to one of the citizens of that country, who sent him into his fields to feed pigs.
16 And he was longing to be fed with the pods that the pigs ate, and no one gave him anything.
17 “But when he came to himself, he said, ‘How many of my father’s hired servants have more than enough bread, but I perish here with hunger!
18 I will arise and go to my father, and I will say to him, “Father, I have sinned against heaven and before you.
19 I am no longer worthy to be called your son. Treat me as one of your hired servants.”‘
20 And he arose and came to his father. But while he was still a long way off, his father saw him and felt compassion, and ran and embraced him and kissed him.
21 And the son said to him, ‘Father, I have sinned against heaven and before you. I am no longer worthy to be called your son.’
22 But the father said to his servants, ‘Bring quickly the best robe, and put it on him, and put a ring on his hand, and shoes on his feet.
23 And bring the fattened calf and kill it, and let us eat and celebrate.
24 For this my son was dead, and is alive again; he was lost, and is found.’ And they began to celebrate.
25 “Now his older son was in the field, and as he came and drew near to the house, he heard music and dancing.
26 And he called one of the servants and asked what these things meant.
27 And he said to him, ‘Your brother has come, and your father has killed the fattened calf, because he has received him back safe and sound.’
28 But he was angry and refused to go in. His father came out and entreated him,
29 but he answered his father, ‘Look, these many years I have served you, and I never disobeyed your command, yet you never gave me a young goat, that I might celebrate with my friends.
30 But when this son of yours came, who has devoured your property with prostitutes, you killed the fattened calf for him!’
31 And he said to him, ‘Son, you are always with me, and all that is mine is yours.
32 It was fitting to celebrate and be glad, for this your brother was dead, and is alive; he was lost, and is found.'”