Simple. By showing them what the Bible says. First, nowhere does the Bible say that Baptism is merely a “symbolic” act…that passage simply does not exist. Second, let’s see what the Bible does say about Baptism: In Ezekiel 36:25-27 it says, “I will sprinkle clean water upon you, and you shall be clean from all your uncleanness…a new heart I will give you and a new spirit I will put within you…and I will put My spirit within you…” Here, in the Old Testament, we have a foreshadowing of New Testament baptism. Now, let’s see if the New Testament corresponds to what we just read in Ezekiel. Acts 2:38, “Repent, and be baptized every one of you in the name of Jesus Christ for the forgiveness of your sins; and you shall receive the gift of the Holy Spirit.” Note that there is no symbolic language here…this is real! The Book of Acts says, “Be baptized for the forgiveness of your sins.” Ezekiel says, “I will sprinkle clean water upon you and you shall be clean from your uncleanness.” The Book of Acts says, “…and you shall receive the gift of the Holy Spirit.” Ezekiel says: “…and I will put My Spirit within you.” Do you begin to see how God, in the Old Covenant, was preparing us for what He gives us in the New Covenant? Acts 22:16 – “And now why do you wait? Rise and be baptized, and wash away your sins…”. 1 Cor 12:13 – “For by one Spirit we were all baptized into one body…” What body was that? The Body of Christ. 1 Peter 3:21: “Baptism, which corresponds to this, now saves you…”  
 
Scripture simply does not support the non-Catholic notion that Baptism is symbolic. Scripture does very directly and very clearly support the Catholic teaching that Baptism saves us; that Baptism makes us members of the Body of Christ; that Baptism washes away sin; and that through Baptism we receive the Holy Spirit…just as the Catholic Church teaches.

The Church’s practice of infant baptism stems from her teachings regarding original sin (what we have) and baptism (what we do about it). After the sin of Adam and Eve in the garden, all people are now born with original sin due to our fallen human nature. Through the gift of grace in Baptism, God washes away this stain of original sin and makes us a part of His family and offers us eternal life. Children, who are born with the stain of original sin, are also in need of Baptism, in order to free them from the bondage of original sin and make them children of God. Our loving Father does not wish to withhold His love and grace from anyone, including children. Baptism simply requires openness. “The Church and the parents would deny a child the priceless grace of becoming a child of God were they not to confer Baptism shortly after birth.” (CCC 1250).