Catholics differ from some Christian Churches which accept the Scripture as the only source of God’s revelation. Catholics have a strong belief in the truth of Scripture, but we also believe in tradition as a way in which God continues to reveal truth to us. Tradition can include beliefs, customs, prayers, and worship, the teaching of popes, bishops, theologians and Church councils. It’s our process of continually reflecting on the way in which the Word of God encounters our own experience as a community of faith.
Catholic understanding is that tradition includes the Scripture, and began before the gospels and letters were written. We do believe that Scripture is a unique revelation from God and that the truths of tradition must always be tested and evaluated against the truths revealed in Scripture. They should not contradict Scripture. They should find their roots in Scripture.
The belief that Mary lived without sin from the moment of her conception springs from Church tradition. It evolved over a period of time, and was not formally defined as a teaching of the Church until 1854. It is not found explicitly in Scripture, but seems for Catholics to flow naturally from the testimony of Scripture that Mary was “full of grace” (Luke 1:28) and “blessed” (Luke 1:42).
In Catholic understanding the belief in Mary’s “immaculate conception” does not say so much about Mary as it is about Christ’s saving power. We believe that God created the human person to be in God’s own image. Grace is more original than sin. Our natural state was to be “full of grace.” Sin is our universal experience but it’s not what God intended for us in the past nor wants for us in the future. We are saved from sin through Christ. Mary’s being conceived without sin takes place in the context of the entire saving act of Christ. In being “full of grace” she is a model of what we human beings were intended to be and who we are redeemed to be through God’s saving power. She is the first sign of God’s victory over sin in Christ.
You also ask: “Why are Catholics who are baptized when they were infants, choose to lead unholy, even atheistic lives, die and then the priest at the funeral says they are going to heaven because they were baptized?”
The bishop of Rome, as head of the Catholic Church, sometimes declares certain people to be “saints” because they lived such exemplary and holy lives that we can believe with confidence that they are “with God in heaven.” Such a declaration only comes after a long investigation into the facts of that person’s life. The Catholic Church has never declared any deceased person to be “in hell.” Aside from this we don’t know and can’t presume the eternal state of any person who has died, although we believe that God desires all people to be with him in death as well as in life. Baptism does not automatically ensure that a person will go to heaven when he or she dies. We must try to live out our Baptism with lives of faith, hope and love.
The Catholic Church asks its members to pray for those who have died BECAUSE we know that we are all sinners, and even when we have tried to live a good life we can die short of the purity of heart that makes eternal life “with God” a reality. Yet we believe that God’s mercy is not thwarted by death and that God has the power to purify and transform the soul even after death so that it is fully open to participating in eternal life. Purgatory is not a place of “sentences”, like a prison, because it exists in eternity which is timeless.
We should certainly pray for the living as well, that they may accept God’s grace into their lives.