I find this quote from a Scottish theologian, Donald Macleod, helpful.
Who has the right to believe? Who has the right to come to Christ? That question has been discussed very thoroughly in Reformed theology and the answer has been unambiguous: every human being, without … exception whatsoever, is entitled to come to Christ and to take Him as his own Saviour. Every man as a man, every sinner as a sinner, the foulest, the vilest, the most vicious—it was put in the strongest possible terms—had the right to come.
This was based on certain clear emphases of the Word of God itself. For example, God commands every human being to believe. No one is exempt from that command. We have the right to come to Christ, whoever we are, because God commands us to come to Christ.
We have the right, secondly, because of God’s offer and invitation to come to Christ. “Look unto me, and be ye saved, all the ends of the earth” (Isa. 45:22); “Come unto me, all ye that labour and are heavy laden, and I will give you rest” (Matt. 11:28); “Let the wicked forsake his way … and let him return to the Lord” (Isa. 55:7). The offer was absolutely universal.
Thirdly, there is a universal divine promise: if we believe, we shall be saved. That is God’s promise. Now it is a conditional promise. The reward is conditional upon our believing. But God’s promise is made categorically: if we turn to God in Christ we shall be saved. Alternatively, it can be put in these terms: the warrant is universal because it arises from the fact that the Bible explicitly states that there is no price to be paid. This salvation is utterly gratuitous (Isa. 55:1). We receive the water of life freely (Rev. 22:17). We take it without money and without price (Isa. 55:1).
Some Reformed preachers went to great lengths to express this fact that every human being, no matter how sinful, has the right to come and take Christ as his Saviour. They were predestinarians of the deepest dye (men like Thomas Boston, John Duncan, and Martin Luther) but they believed equally firmly in the free, universal offer of the gospel. John Duncan put it most succinctly: “Sin is the handle by which I get Christ.” [He went on,] “I don’t read anywhere in God’s Word that Christ came to save John Duncan … but I read this: He came to save sinners and John Duncan is a sinner and that means he came to save John Duncan.” Luther argued in the same way. He said to the devil, “Thou sayest I am a sinner. And I will take thine own weapon and with it I will slay thee and with thine own sword I will cut thy throat because sin ought to drive us not away from Christ but towards Christ.” The Bible and Reformed theology have taught us to come—just as we are.
Just as I am, and waiting not
To rid my soul of one dark blot,
To Thee, whose blood can cleanse each spot,
O Lamb of God, I come.
Now it may be that in Reformed theology there is no theological answer to the question, “How can it be simultaneously true that only the predestined are saved and that God commands all men to believe?” All we can say is that both horns of the dilemma are equally valid. For the moment our concern is with only one aspect of the truth: every human being is warranted to come to Christ. The great thing here is that the universal becomes … particular. If all are warranted, each is warranted. If each is warranted, I am warranted. This is supremely important in relation to those who are tempted to spiritual despair: the backslidden, those who were once bright, shining Christians, but from whose lives the glory has gone and who feel that for them there is no hope. Wherever we stand, we have the warrant to believe.
From a sermon, “Amazing Grace”, by Alistair Begg.