For the beginning verses of Genesis 25, Pastor Voddie spoke about the life of Isaac and his somewhat minimal narrative in the Bible. With Isaac, his importance is defined in terms of who he's connected to in terms of God's covenantal promise.
(Voddie touched on the "controversy" around Gen. 25:1 and his other offspring, and what was the situation that empowered him to have more children. Was it his "rejuvenation", that allowed him to have Isaac, also something that allowed him to have other children as well? The issue with Keturah and whether she was a wife or concubine is interesting but ultimately not important. One interesting note is that Keturah bore Midian, and later on with will be the Midianites who took Joseph out of the well to sell to the Ishmaelites.)
The various promises that shape the life of Isaac include God's promise to provide offspring for barren Abraham, as well as the land of Caanan. We also see through God's promise that Ishmael would be a great nation an example of Abraham's unfaithfulness, and presents the account of another son who does not get to be the promised one. Abraham did not have to sin to fulfil the promise. Ishmael was set against Isaac, and this is still manifest to this day.
In terms of the messianic linage, through Abraham, Isaac, Jacob, etc, we see that it wasn't from the goodness and sinlessness of these individuals that God called them to be of the chosen line. Rather, God used this lineage of sinful men to lead up to Christ, the one through whom forgiveness of sins would be offered.