The Bible is clear that our ability to believe in God--that Jesus is Lord, is an ability granted by God Himself and not something we are able to do by ourselves . . .
"29 For to you it has been granted for Christ’s sake, not only to believe in Him, but also to suffer for His sake" --Phil. 1:29
. . . and that this is how we are saved:
"If you declare with your mouth, 'Jesus is Lord,' and believe in your heart that God raised him from the dead, you will be saved." --Rom. 10:9
And it is clear that just as the numbers of hairs on our bodies are known and counted by God Who determined that they should be so, so is the number known, counted and determined who are chosen by God to be saved--those whom He has "granted . . . to believe in Him."
Most street preachers evidence this when they often tell a prospect something like, "I can't make you believe--the Holy Spirit is the One who must convict you. . . . I can only tell you the truth and then it will be the Holy Spirit Who will open your eyes or not. . . ."
So then some will say, "So why bother? If it has already been decided who will and who will not believe, what is the point of preaching, evangelizing at all?"
Well, the point is always about the demonstration of God's glory--which is what He is doing here with all of this; and what is to be grasped is that He has also determined to work through fallible, fallen mankind to effect His will in an era of space and time--in this creation. "But God chose the foolish things of the world to shame the wise; God chose the weak things of the world to shame the strong." --1 Cor. 1:27
Demonstration and witness. . . . This is what it's all about--at least so far as has been revealed. The Lord is demonstrating His magnificent, glorious attributes and made us to be witnesses and participants in that demonstration.
Could God have chosen to create a world where all get saved? Yes, of course. He could have created a world where none sinned, as it will be in Heaven. But He did not; for His own "good pleasure" (Eph. 1:9) He has planned it the way He wills, and "who are you, a mere creature, to question God?!" (Rom. 9:20)
". . . the true task of the "Church" [is to] proclaim the Gospel that we become a savor of life unto life in them that believe and a savor of death in those who reject the truth." --Barnhouse
Knowing this, we need not fret nor be anxious nor fear that our preaching of the Gospel may not be "effective". . . . We can rest assured, in the peace of God, that those who are to be granted to believe WILL believe, either now or in time . . . and that those who reject the Good News, in the face of our witness, do so that their "cup of iniquity" should be filled!
As witnesses--as actual "ambassadors for Christ, as though God were making an appeal through us" (2 Cor. 5:20) our expression of the truth we have been shown acts to give strength, hope, "savor of life" to the "believing ones" (see John 3:16, properly translated) while compounding just wrath upon the heads of those who continue in rebellion. This is our role as witnesses, as ambassadors, as "temples of God" wherein the Spirit of God dwells (1 Cor. 3:16).
In this sense we are "prophets" who "admonish" in the name of Christ, "priests" who teach, give comfort and encourage other believers. . . . (see Col. 3:16, Rom. 15:14, Heb. 3:13)
We are called to be witnesses of the truth before believers and unbelievers alike, not so much to persuade but to declare . . . so that those chosen will hear and be saved . . . or so that the already saved will be encouraged . . . or so that the children of the devil are further condemned in their refusal to submit.
In this way, by means of we creatures, on the stage of the world, in this space and time, the Word of God goes out and does not "return void" but in every case, "accomplishes" all that the Lord intends.
(see Isa. 55:11)
God speaks through His witnesses, demonstrating His glory, encouraging or compounding wrath, and all His will is done. . . .