Why do we play classical music or sing to babies or tell them stories when they do not understand yet? Because everything in those formative years has a profound influence on the child.
With baptism, it is no different. We are telling the baby the truth. But we are not simply telling them, we are showing them. Enacting the truth. The water represents death, and the baby is fully immersed and lifted - to enact the resurrection. We are demonstrating the truth about the reality of things. Obviously, the baby is not aware in the present moment of what is happening, but it now grows up with the knowledge that it has been baptised. And that is the difference. That is key. No matter who or what you become in life, you always have this knowledge that you have been baptised. Initiated into a group called the Christians. Whether you like it or not, that forms part of your history. Your identity. It’s a niggling whisper. A dormant seed. And perhaps it will lead you to the light one day. We as parents tell the child the truth, we believe it is so important we tell them and enact it when they are newborn. It is up to them if they accept it or not.
On top of all this, baptism is one of the church’s “Mysteries”. We call it a mystery, because we literally have no idea what is happening spiritually. But we do know it is good.