There are many elements that are a part of the standard baptismal liturgy in the Lutheran church. There are prayers. There is the confession of the Creed. There’s a blessing. There is often a white garment and often a candle. There are usually sponsors and/or parents. There is, of course, the pastor and the Christian being baptized.
In and among the different elements of the baptismal rite is the sign of the cross. The pastor briefly marks the one to be baptized with a cross over the head and over the heart. It comes and goes quickly and probably doesn’t mean much in the moment, but of the elements of the rite, it is one whose effects can be seen for much longer. I say that because you probably won’t wear white every day, or even every Sunday. You aren’t going to walk around with a lot candle wherever you go. You aren’t going to be welcomed by the congregation on a weekly basis. However, every Sunday is a Sunday you are baptized. Every Sunday, indeed every day, is one where you have been claimed by God and where you are covered in the righteousness of Christ, the Crucified.
The early church would often talk about baptism as conveying an “indelible seal.” Baptism marks you in a way that nothing in creation has the power to remove. God has claimed you, forever and always and He will never change His mind. Everything God offers in baptism is yours in perpetuity.
Whether you choose to mark yourself with the sign of the cross does not change your standing before God. It does not give you any more grace or other blessings from God because that was all given to you already in your baptism. What it does do is act as a constant reminder of who you belong to and how you came to be His.
You were baptized in the name of the Triune God: Father, Son, and Holy Spirit. You have been adopted into His family and been given His name. The sign of the cross becomes a way of looking at your adoption certificate, always there to remind you that, however you feel and whatever you may have done, you are part of His family. Luther suggests making the sign of the cross as you say your morning and evening prayers. That way, the whole day through you will remember you are covered by that baptismal promise. Making the sign of the cross during church also reminds you that, whether you are receiving forgiveness, whether you are receiving any of the other blessings of God, whether you are confessing your faith, or really just about anything else, these things are all yours because you have been baptized and that all of this is yours throughout your life and into eternity.