"Come, all you who are thirsty, come to the waters..." - Isaiah 55:1
When a person is thirsty, he needs a drink. God calls us to come and drink from his everlasting fountain of grace. Though this is a basic physical need, this verse points to a deeper sort of thirst - a spiritual thirst. God wants his people to be satisfied (v. 2). Later in the passage the Holy Spirit through Isaiah says, "Seek the LORD while he may be found; call on him while he is near" (v. 6). Why seek God? He says, "Let us turn to the LORD, and he will have mercy on him, and to our God, for he will freely pardon" (v. 7).
As Matthew Henry says, the qualification for those that shall be welcomed to drink is that "they must thirst." He continues, "Those that are satisfied with the world and its enjoyments for a portion, and seek not for a happiness in favour of God, -- those that depend upon the merit of their own works for a righteousness, and see no need they have of Christ and his righteousness, -- these do not thirst; they have no sense of their need, are in no pain or uneasiness about their souls, and therefore will not condescend so far as to be beholden to Christ. But those that thirst are invited to the waters, as those that labour, and are heavy-laden, are invited to Christ for rest."
As long as a person is not thirsty, he may not realize that he needs a drink. If a person is thirsty, he knows that he needs a satisfying drink. Fortunately, there is a never-ending Fountain available to eternally satisfy this need - Christ.
Are you thirsty? "Come, all you who are thirsty" and drink!