In Isaiah 55:1, the God of the Universe, who owns everything, sent out an invitation to a specific group of people: to those who were thirsty (parched) and who were flat busted, broke.
The reason they had no money was because they had spent it on all the wrong things. (v.2). Yet God simply extends an invitation to them - come, buy without money and eat without cost. It was the only type of invitation they could have responded to. No formal, black tie dinner invitation would have done them any good because they couldn't afford the price - they couldn't buy themselves fancy apparel and look the part.
Jesus extended the same invitation as the one in Isaiah: "Jesus stood and cried out, saying, 'If anyone is thirsty, let him come to Me and drink'." In Rev. 21:6 He said, 'It is done. I am the Alpha and the Omega, the beginning and the end. I will give to the one who thirsts from the spring of water of life without cost.'
Caring for the sick, working for the church, reaching out to those in need - those are good things, things we ought to do, and they please God. But they don't impress Him.
Why? Because God looks on our hearts. Anything less than total purity, unadulterated love, and pristine worship inside us contaminates any good works that we might do. Any momentary thought of, "Look how good I am for doing this..." - that's pride. God says over and over through the Bible that He hates pride, that He is opposed to pride, that pride is what caused Satan to fall from Heaven...
So unless our thoughts and motivations are perfectly pure all the time, we simply don't have the coin of Heaven, we don't have the wherewithal to impress God or to buy (work) our way into Heaven.
But then, when we look into Scripture, we get this amazing invitation: Come, all of you, who are thirsty and broke, come...
ANd what is the "food and drink" that God offers in Isaiah? True, total, and complete forgiveness - a forgiveness like nothing that can be found in the heart of man, a forgiveness that can never be earned,a forgiveness that makes us clean and whole in the eyes of a perfect, sinless God, giving us a clean slate that will never be dirtied again in God's eyes. (Isaiah 55:6-11)
A forgiveness that draws us to our knees in humility and gratitude and then lifts us to our feet to serve, not out of compulsion and endless "trying harder", but out of adoration for the One who loved us so much.
Who wouldn't want to serve a Giver like that???