Is a person a Kafir if he doesn't offer a two Rakah prayer after having a bath although he believes in all articles of faith Islam when he became Muslim? If a person becomes Muslim but didn't know he needs to offer a bath (Ghusl) and a two Rakah prayer and its been a very long time, does all the good deeds he has done be rejected by Allah because when he first became Muslim he didn't take a bath and didn't offer a two Rakah prayer due to the fact that he didn't know you had to?

Praise be to Allah, the Lord of the World; and blessings and peace be upon our Prophet Muhammad and upon all his Family and Companions.
Pronouncing the testimony of faith i.e. La Ilaha Illa Allah Muhammad ur-Rasoolullah (there is none worthy of worship except Allah and Muhammad (Salla Allahu Alaihi Wa Sallam) is Allah's Messenger), and firmly believing according to this testimony without having any doubt in it makes one a Muslim. So, taking a bath or performing two Rakah are not pre-conditions for the validity of entering Islam. Without doing so a person is considered a true Muslim. However, the majority of the Muslim scholars hold the opinion that taking a bath and performing two Rakah after pronouncing testimony of faith is desirable.
One should know that faith (Iman) consists of many branches and every branch is called Iman (belief). Therefore, prayer is part of belief, Zakah is a part of belief and all other forms of worship and good acts are considered from belief. The deeds that are peculiar to the heart such as, trust in Allah, and Haya (self-respect, bashfulness) etc. are also part of faith. Some of these branches of faith are so essential that if they are abrogated the faith itself is nullified such as the branch of pronouncing testimony of faith. Some of them are not so important such as removing what is harmful from the road, which is also part of the faith. Between these two branches of faith there are many different grades. Some are as important as the branch of testimony while others are not so important such as the act of removing harmful things from the road.
Therefore, whoever neglects desirable acts, Sunnahs, or even some recommended or obligatory acts or commits some offensive deeds, such a person is not considered an apostate. He is still a Muslim but one who commits sins or a dissolute Muslim.
Finally, a Muslim is always demanded to raise himself to higher ranks by following all the rules and etiquette of Shariah. When he excels he will become near to the rank of righteous people and near to Allah as well.
Allah knows best.