Here’s the link to Article XIII: https://bookofconcord.org/augsburg-confession/use-of-the-sacraments/
Essentially, there are three points in this article:
1. When we receive the sacraments, it is indeed a public testimony of faith. However, the sacraments are much more than that. They are a gift of grace from God to us, and they have been given to us in order to awaken and confirm faith.
Baptism: When you are baptized, the world sees and knows that you are now a Christian. It’s a “mark of profession among men.” But beyond this, baptism is God’s gift to us. It unites us with Christ (Galatians 3:27; Romans 6:4), cleanses from sin (Acts 2:38-39), gives rebirth and renewal (John 3:3-5; Titus 3:4-7), and makes us part of the church (1 Corinthians 12:12-13).
Holy Communion: Every time you walk to the front of the church to receive the bread and wine, you declare to the church that you believe in Jesus. It’s a “mark of profession among men.” But it’s more than that. Holy Communion is a “participation” in Christ’s body and blood (1 Corinthians 10:16-17). The words “this is my body” and “this is my blood” (Mark 14:22-24) should be taken at face value. Jesus is really present in the sacrament.
2. The benefits bestowed in the sacraments are received by faith. A person can be baptized externally, but if they do not believe the gospel, they will be condemned (Mark 16:16). Likewise, a person who eats the Lord’s Supper “unworthily” does so to to their own harm (1 Corinthians 11:27-29).
3. Therefore, the outward act of receiving a sacrament does not in itself justify a person: faith in Christ is necessary. This reminds us of how important it is to continue instructing baptized people in the faith (see Matthew 28:19).
A few other comments:
– Some Christians raise the question: what if I didn’t believe at the time I got baptized, but I do believe now? Should I get rebaptized? The answer is no. God’s work in baptism was effective then and it’s still effective. Repent of your unbelief and trust in him.
– Private Confession and Absolution is another resource that’s available to us. In fact, Lutherans occasionally speak of Confession and Absolution as a third sacrament. Melanchthon does so in his defense of the Augsburg Confession (https://bookofconcord.org/defense/of-the-number-and-use-of-sacraments/). However, Confession and Absolution lacks an outward visible sign. Hence, we more often think of it as a personalized application of the gospel.
Christ Alone Lutheran Brethren Church 