The Catholic Encyclopaedia states that believers are “to worship God and then to pray, first to Saint Mary, and the holy apostles, and the holy martyrs, and all God's saints, that they would intercede for them to God. In this way they learned to look up to the saints in heaven with feelings of confidence and affection, to consider them as friends and protectors, and to implore their aid in the hour of distress, with the hope that God would grant to the patron what he might otherwise refuse to the supplicant.
This is not biblical at all and was not taught by any of the prophets. This is an invention of the Catholic Church. These saints are avatars of pagan gods.
As mentioned in the section ‘Christianity becomes the state religion of Rome’, pagan statues (of gods) were relabelled.