Genesis 38

Self-righteousness has a way of coming back on us

Judah was not ignorant of the command given to his father and grandfather not to marry into the Canaanites. He finds a friend in Hirah, and while hanging out with him, instead of his own family, he finds a Canaanite woman, Shuah and marries her. With Shuah he has three sons, Er, Onan and Shelah. Er marries a woman named Timnah and lived his life in evil the bible says; so God slays him. Whether he is slayed by his own evil living or just outright struck down by God, I suppose it ends the same. Following Jewish custom, Onan is now supposed to marry her and raise up children in his brother Er's name. Onan wants no part of this and refuses to give her children in her brother's name and God slays him as well. 

Judah intervenes and tells Timnah to stay at her father's until Shelah is old enough to be her husband. Time passes and is so often the case, we forget our vows and promises and during this time Judah's wife dies. While trying to forget his sorrow he decides to go into a harlot, but the surprise is on him because he unknowingly lays with and impregnates his daughter-in-law who laid in wait for him. 

After time passes and word comes back to him that Timnah is pregnant, Judah, who hasn't lived a life for God since he left his family to go live with the Canaanites, gets fired up and says that his sinful daughter-in-law must be put to death to pay for her 'sins'. Timnah has a secret for Judah and when she presents him with the signets he left as a token to pay her for their rendezvous, he is immediately humbled and says out loud that she has been more just than he has.  

Timnah ends up having twins, Pharez and Zarah. During delivery Zarah puts him arm out and he is marked with a scarlet thread to show he was the first born, which is important to Jewish law and tradition. Astonishingly though, Pharez ends up coming out first. Pharez will become important because it is through his line that kind David and the Kind of Kings comes. The circumstances that brought you into this world do not matter as much as what you do in this world once you are here.