Free will

Rev. 3:20 Behold, I stand at the door, and knock: if any man hear my voice, and open the door, I will come in to him, and will sup with him, and he with me.

Acts 2:39 For the promise is unto you, and to your children, and to all that are afar off, even as many as the Lord our God shall call.

Acts 2:38 Then Peter said unto them, Repent, and be baptized every one of you in the name of Jesus Christ for the remission of sins, and ye shall receive the gift of the Holy Ghost.

Romans 6:4 Therefore we are buried with him by baptism into death; that like as Christ was raised up from the dead by the glory of the Father, even so we also should walk in newness of life.

To be born again we have to open the door, accept the promise and be baptised. It does take some doing on our part. "many are called"