"God is not a man, that he should lie, nor a son of man, that he should repent." Numbers 23:19
These words are spoken by Balaam to Balak, king of Moab. Balak brought Balaam to his land to curse the Israelites. Balaam has been commanded to speak evil of Israel. We know that Israel consists of sinners, guilty sinners, and yet the Lord continually gives Balaam only blessings to say. They are the Lord's people, regardless of their internal state. His righteousness covers them, covers their sin, just as snow covers the black ground. Again and again Balaak tries to get this prophet to curse them, and yet he can only bless them.
In the midst of his second blessing, Balaam says that "God is not a man, that he should lie, nor a son of man, that he should repent." From this we learn
1) God is sin-free. In him is light and no darkness at all. We, on the other hand, are sinners. We have darkness, and this darkness does not allow us access into his heavenly presence. However, he sent his son, Jesus Christ, to pay the penalty for this darkness in us, as if he himself were the sinner. Jesus was the perfect man, the blemish-free lamb, without spot or fault, and so his death satisfies the payment for the sins of all of God's elect. God's righteousness is imputed to us, and our sin is imputed to God. This is why it is a good thing that God does not lie, and there is no guile in him. Otherwise, how can he give us his righteousness?
2) God does not repent, nor does he need to, because he makes no mistakes, and once again, he has no sin. But we need to repent, and it is the hardest thing to do in the world, because our indwelling sin prevents us from repentance. Too much pride remains within us. However, Christ intercedes on our behalf for us, and his Spirit repents for us, bring our full selves to repentance. We are weak and unable to repent. We cannot even pray as we should. This is why Christ intercedes for us, because we cannot do it ourselves. Christ prayed perfectly, and he taught us how to pray, even though he did not need to pray, "forgive us our sins." His prayers, like his righteousness, is transferred to us, as our sinful selfishness is imputed to him.
3) John tried to prevent Jesus from undergoing his baptism of repentance in the river Jordan, but Jesus answered him, "Permit it at this time; for in this way it is fitting for us to fulfill all righteousness. (Matthew 3:15)" Why did Jesus undertake a baptism of repentance? He was sin-free. He had nothing to repent of. But we, his children, have much to repent of, and so, like his death, his baptism of repentance is applied to us, as our sin is applied to him. This baptism, like his other obedience to the law, is part of his righteousness, and all of his righteousness is applied to his elect, not just some. To fulfill all righteousness is to fill all of the righteousness to its maximum capacity, leaving nothing out. All of Christ's righteousness gets applied, so that the Father can welcome us into his kingdom as glorified, little Christs who are covered by his sanctified blood.
God did not need to send his Son to die on our behalf. The incarnation was not necessary, but it was God's good pleasure to save his elect and thereby show his glory to the world. God doesn't need us. He made us for himself, but he is not empty without us. The incarnation is God choosing to save his people, even though he didn't need to. Likewise, the Crucifixion was not necessary. He could have allowed all mankind to be lost to eternal death forever, but it was his good pleasure to save his people from eternal ruin. Finally, he doesn't need to intercede for us, to continue us on a path of righteousness, to repent on our behalf, and yet he does, so that we can be sanctified in life. All three of these things show God's great love for his people. He didn't need to do any of these things, but he chose to, because he loves us, and he never lies. He spoke in his Word that it is his will to save us, and to not let one of us be lost. He speaks the truth.