Baptism

Lutherans believe that, in baptism, a person is born into the Kingdom of God and becomes an heir of salvation. It is the beginning of life of faith in which each day our human nature 'should be drowned through daily repentance; and that day after day a new self should arise to live with God in righteousness and purity forever." (The Small Catechism, by Martin Luther)

Religions in America, p. 116

The Apology [Defense] of the Augsburg Confession Article IX: Of Baptism.

51] The Ninth Article has been approved, in which we confess that Baptism is necessary to salvation, and that children are to be baptized, and that the baptism of children is not in vain, but is necessary and effectual to salvation. 52] And since the Gospel is taught among us purely and diligently, by God's favor we receive also from it this fruit, that in our Churches no Anabaptists have arisen [have not gained ground in our Churches], because the people have been fortified by God's Word against the wicked and
seditious faction of these robbers. And as we condemn quite a number of other errors of the Anabaptists, we condemn this also, that they dispute that the baptism of little children is unprofitable. For it is very certain that the promise of salvation pertains also to little children [that the divine promises of grace and of the Holy Ghost belong not alone to the old, but also to children]. It does not, however, pertain to those who are outside of Christ's Church, where there is neither Word nor Sacraments, because the kingdom of Christ exists only with the Word and Sacraments. Therefore it is necessary to baptize little children, that the promise of salvation may be applied to them, according to Christ's command, Matt. 28, 19: Baptize all nations. Just as here salvation is offered to all, so Baptism is offered to all, to men, women, children, infants. It clearly follows, therefore, that infants are to be baptized, because with Baptism salvation [the universal grace and treasure of the Gospel] is offered. 53]

Secondly, it is manifest that God approves of the baptism of little children. Therefore the Anabaptists, who condemn the baptism of little children, believe wickedly. That God, however, approves of the baptism of little children is shown by this, namely, that God gives the Holy Ghost to those thus baptized [to many who have been baptized in childhood]. For if this baptism would be in vain, the Holy Ghost
would be given to none, none would be saved, and finally there would be no Church. [For there have been many holy men in the Church who have not been baptized otherwise.] This reason, even taken alone, can sufficiently establish good and godly minds against the godless and fanatical opinions of the Anabaptists.

Note: The Anabaptists were a group that taught rebaptism as an adult was necessary because an infant was not able to choose to be baptized. These people were persecuted by Protestants and Catholics alike.