Isaac did not reject Esau, but the way things worked out Isaac realised that it was the overruling of God that had led to Jacob supplanting Esau. Isaac trembled once he discovered what had happened, because he realised that God had intervened.
If we do not respect God, and reverence him, and seek him, and long for him, and trust him for grace now, then when the time comes when we would have inherited the blessing, then it will be too late. He will not change his mind. This fearful scene is set before us by the Lord Jesus Christ: ‘Many will say to me in that day, Lord, Lord, have we not prophesied in thy name? and in thy name have cast out devils? and in thy name done many wonderful works? And then will I profess unto them, I never knew you: depart from me, ye that work iniquity’ (Matthew 7:22-23). We can call upon him in the hour of death as we cross into eternity and say, ‘Oh Lord, save me now’, but he will not change his mind and we will be condemned. That is what we should learn from this passage, not that we can be rejected even now even though we sincerely repent. So it is Isaac’s repentance that is in mind. It is the birthright that Esau sought carefully with tears, not repentance on his own part.
Nevertheless, there is also such a thing as wrong repentance in which we are not sorry for our sin but for the consequences that our sin has brought upon us. We may sense a loss of God's blessing upon our lives. We may sense that we are in trouble because of our sin, and we repent in order to be reinstated into the favour of God, but we do it not because we are sorry for our sin. It is possible some people even come to Christ on this basis. They tremble out of fear of going to hell, so they repentant of what might happen to them. Well it is good for people to be driven to a true repentance by fear of punishment and hell as you have got to repent of the sin, not just seek to avoid the punishment and sorrow over that.