The Levitical priests of ancient Israel were required to be without physical defect and ritually pure before performing their Temple or Tabernacle duties. A descendant of Aaron had to be pure, unspotted, and without physical defects such as blindness, lameness, or disease before offering sacrifices to God. The priest could not approach the altar or go near the veil if he had a defect, as it would profane God's sanctuary. When the Temple in Jerusalem was completed in 515 BC, the priests purified themselves according to God's instructions in Leviticus, achieving ritual cleanliness through sin offerings and obedience to His law. In the New Testament church, God calls a spiritual royal priesthood, where purity remains essential. The ritual purity of the Levitical priesthood involved physical acts like washing and sacrifices, while the spiritual purity of God's royal priesthood requires cleansing of the mind and attitude. Purification is an ongoing process, as the blood of Christ's sacrifice initially washes away sins at baptism, but continual effort is needed to rid the mind of sin. Pure and undefiled religion before God involves keeping oneself unspotted from the world, unblemished by sin. The purifying of thoughts comes through studying God's Word and applying it in life, replacing carnal thoughts with righteous, pure ones. Purity is the spiritual cleansing of the mind, resulting in complete devotion to God. By obeying the truth, souls are purified, fostering sincere love among brethren. Everyone who holds the hope of Christ's return purifies himself, just as He is pure. As God's royal priesthood, it is our duty to prepare for His Kingdom by purifying our minds with His holy words and thoughts.