Genesis 38:1 says, And it came to pass at that time, that Judah went down from his brethren, and turned in to a certain Adullamite, whose name was Hirah.  Jesus was born from the house, or descendants, of Judah.  This does not seem a likely family for the Messiah to be born from, as we shall see.  Verse two states,  And Judah saw there a daughter of a certain Canaanite, whose name was Shuah; and he took her, and went in unto her.   While separated from his family, Judah took the daughter of a Cannanite as his wife.  Matthew Henry says that Judah married too young and that his marriage was arranged by his wife's father and not by his own father, which was customary.  I believe that the main point is that he did not marry a woman who was also a part of God's chosen people.  Verse three declares, And she conceived, and bare a son; and he called his name Er.  Judah had a son, who was named Er.  Verse four adds,  And she conceived again, and bare a son; and she called his name Onan.  Then verse five concludes,  And she yet again conceived, and bare a son; and called his name Shelah: and he was at Chezib, when she bare him.  Judah now had three sons, and he was at least carrying out God's promise to Abraham, Isaac and Jacob that he would make a great nation from them, but he was not carrying it out under God's authority.  As followers of Christ, we cannot just do what we want to do and expect God to bless us, but we must seek His will first.  Verse six states, And Judah took a wife for Er his firstborn, whose name was Tamar.  Judah chose a wife for Er.  Her name was Tamar.  Matthew Henry says that Er was too young to be married, but I believe that it was his character and not his age that was a problem.  Verse seven declares, And Er, Judah’s firstborn, was wicked in the sight of the LORD; and the LORD slew him.   As just stated, Er was a wicked man, not necessarily in the eyes of the world, but in the eyes of God.  We are told that God slew Er.  I am not sure how this happened, but we do know that God is the Creator and sustains life, so however Er died, people saw it as God killing him.  Er must have been particularly evil for this to happen.  Verse eight states, And Judah said unto Onan, Go in unto thy brother’s wife, and marry her, and raise up seed to thy brother.  As was the custom, when Er died Judah had Er's brother Onan to marry Tamar to preserve Er's name by having a child with her.  I am not sure how we would we feel about marrying our brother's widow simply so his name could be preserved, but I believe that like Onan we would not be wild about the idea.  Verse nine adds, And Onan knew that the seed should not be his; and it came to pass, when he went in unto his brother’s wife, that he spilled it on the ground, lest that he should give seed to his brother.  Onan decided that since the child would be viewed as a descendant of Er and not of Onan himself that he would not carry out this obligation.  Verse ten declares, And the thing which he did displeased the LORD: wherefore he slew him also.  We are told that God was not pleased with Onan and killed him also.  I really don't believe that God kills people just because they displease Him, but think it more likely that since Er and Onan both died young that it was attributed to God killing them.  I base this on the nature of God not being vindictive, but this is still just my interpretation.  Verse eleven states, Then said Judah to Tamar his daughter in law, Remain a widow at thy father’s house, till Shelah my son be grown: for he said, Lest peradventure he die also, as his brethren did. And Tamar went and dwelt in her father’s house.  Judah's youngest son, Shelah, was too young to marry, so Judah instructed Tamar to remain a widow in her father's house until Shelah was old enough to marry.  This was all being done to preserve the name of Er by giving him a descendant.  I don't believe that I would have been happy to be either Shelah or Tamar.  We are not told that any of this was God's plan.  As followers of Christ, we cannot make our own plans to carry out God's purpose without ensuring that we are following His will and hope for success.