It all comes from the instructions for 'purification after birth in Leviticus 12:
“Speak to the Israelites. Tell them, ‘Suppose a woman becomes pregnant and has a baby boy. Then she will be “unclean” for seven days. It is the same as when she is “unclean” during her monthly period. On the eighth day the boy must be circumcised. After she has waited the required number of days to be made pure, she must bring two offerings. She must take them to the priest at the entrance to the tent of meeting. She must bring a lamb a year old for a burnt offering. She must also bring a young pigeon or a dove for a sin offering. The priest must offer them to the Lord. They will pay for her sin. Then she will be “clean” from her bleeding.
And again he says, “Here I am. Here are the children God has given me.” '
They are flesh and blood and so too has Jesus - who 'became like them to die for them (us)'. God becomes man (that's who Anna and Simeon see) and does so to die for us. This is the plan to reconcile us to God (Father, Son and Holy Spirit) Jesus, in dying and rising from the dead breaks the power of death, which is the penalty for sin, and in doing so removes all the power that satan has and takes control of his kingdom. This is the place where we understand what the power of the cross truly is!
The Lord who rules over all says, “I will send my messenger. He will prepare my way for me. Then suddenly the Lord you are looking for will come to his temple. The messenger of the covenant will come. He is the one you long for.” But who can live through the day when he comes? Who will be left standing when he appears? He will be like a fire that makes things pure. He will be like soap that makes things clean. He will act like one who makes silver pure. And he will purify the Levites, just as gold and silver are purified with fire. Then these men will bring proper offerings to the Lord. And the offerings of Judah and Jerusalem will be acceptable to him. It will be as it was in days and years gone by.
“So I will come and put you on trial. I will be quick to bring charges against all of you,” says the Lord who rules over all. “I will bring charges against you sinful people who do not have any respect for me. That includes those who practice evil magic. It includes those who commit adultery and those who tell lies in court. It includes those who cheat workers out of their pay. It includes those who treat widows badly. It also includes those who mistreat children whose fathers have died. And it includes those who take away the rights of outsiders in the courts.