What is the meaning of purity in order to respond to the People of the Book concerning that, and why do we purify ourselves for prayer?

Indeed, whoever knows the Islamic Sharia (definition) will not find its rulings problematic, for our trust in God is greater than a patient's trust in his doctor. Purity (Tahara) is legislated for many reasons, including: its conformity with human nature, Islam being a religion of cleanliness and beauty, protecting its practitioner from diseases, preparing for the encounter with Allah the Almighty, and the fact that ghusl (definition) (ritual bath) after major impurity restores strength to the body and replaces what has deteriorated from it – a matter understood by every person of sound intellect. Furthermore, outward purity is linked to purifying the inner self from bad morals, and both are a cause for God's love. Purity removes sins and purifies the body and heart, preparing the one performing ablution to enter before his Lord.