However Jesus said "And I have other sheep that are not of this fold. I must bring them also, and they will listen to my voice."
Is Jesus just going out calling whatever sheep will respond to Him or does He have specific sheep that WILL listen to Him that He WILL bring into the fold?
Well, this is a little complicated, because God's foreknowledge comes in here. God in time does not know you until you believe. But outside of time in God's foreknowledge he knows in advance who will listen.
Jesus knew from the beginning who would believe and who would believe not.
John 6:63 It is the spirit that quickeneth; the flesh profiteth nothing: the words that I speak unto you, they are spirit, and they are life.
64 But there are some of you that believe not. For Jesus knew from the beginning who they were that believed not, and who should betray him.
What does Jesus mention first here? He mentions his words. But then he says there are some of you that believe not, and then it says Jesus knew from the beginning who they were (he knows individuals) that believed not and who would betray him.
When Jesus picked the tweleve disciples, he knew from the beginning that eleven would hear and believe his words, and that one would not.
John 6:70 Jesus answered them, Have not I chosen you twelve, and one of you is a devil?
71 He spake of Judas Iscariot the son of Simon: for he it was that should betray him, being one of the twelve.
Now this is Jesus showing foreknowledge. He knows who will listen and believe him, and he knows who will not beforehand.
That is God outside time, that is foreknowledge. God outside time knows everything that will happen in the future. This is why God can tell us future events and give us prophesy.
But at other times the scriptures show God inside of time. God does not know you in a very close personal relationship until you trust on Christ.
1 Cor 8:3 But if any man love God, the same is known of him.
When you read this verse, it shows that to be known of God is conditional. You must love God to be known of him. This is speaking of a saved person, one who has trusted Christ.
When Jesus spoke of the Gentiles, he was saying he has other sheep. And he identifies who they are, they are those who will listen to and believe his words.
John 10:16 And other sheep I have, which are not of this fold: them also I must bring, and they shall hear my voice; and there shall be one fold, and one shepherd.
So, this is very difficult to explain. Outside of time in God's foreknowledge he knows in advance who will hear his voice. But inside of time he only knows a person once they hear his voice.
We see this in the story of Nathaniel in John 1. Philip had met Jesus and believed him to be the Messiah. He went and found Nathaniel. Nathaniel had faith, he believed and was looking for the savior. When he came to Jesus, Jesus knew him which surprised Nathaniel greatly. But Jesus said he saw Nathaniel even before his brother Philip came after him.
John 1:43 The day following Jesus would go forth into Galilee, and findeth Philip, and saith unto him, Follow me.
44 Now Philip was of Bethsaida, the city of Andrew and Peter.
45 Philip findeth Nathanael, and saith unto him, We have found him, of whom Moses in the law, and the prophets, did write, Jesus of Nazareth, the son of Joseph.
46 And Nathanael said unto him, Can there any good thing come out of Nazareth? Philip saith unto him, Come and see.
47 Jesus saw Nathanael coming to him, and saith of him, Behold an Israelite indeed, in whom is no guile!
48 Nathanael saith unto him, Whence knowest thou me? Jesus answered and said unto him, Before that Philip called thee, when thou wast under the fig tree, I saw thee.
49 Nathanael answered and saith unto him, Rabbi, thou art the Son of God; thou art the King of Israel.
You see verse 48? Jesus said even before Philip came to Nathaniel that Jesus saw him. This is God's foreknowledge. Jesus knew Nathaniel would believe. And he did.