Where do evangelicals get the idea that the point of baptism is to testify outwardly about one’s faith? Not one verse in the Bible suggests that public testimony of one’s own faith is even one of the reasons to be baptized, let alone the chief or only reason. Try it yourself: search for every occurrence of bapt* in the Bible, and see if even one verse in that search suggests that one reason for baptism is for the one baptized to testify of his own faith before other men.
What do you find instead? Ananias saying to Saul of Tarsus, as St Paul later recalled,
The God of our fathers hath chosen thee, that thou shouldest know his will, and see that Just One, and shouldest hear the voice of his mouth. For thou shalt be his witness unto all men of what thou hast seen and heard. And now why tarriest thou? arise, and be baptized, and wash away thy sins, calling on the name of the Lord.
This is why in the Nicene Creed we confess, ‘I acknowledge one Baptism for the remission of sins.’
You will also find St Paul saying of baptism in Romans 6,
Know ye not, that so many of us as were baptized into Jesus Christ were baptized into his death? Therefore we are buried with him by baptism into death: that like as Christ was raised up from the dead by the glory of the Father, even so we also should walk in newness of life.
And St Peter says this of baptism in 1 Peter 3:
For Christ also hath once suffered for sins, the just for the unjust, that he might bring us to God, being put to death in the flesh, but quickened by the Spirit: by which also he went and preached unto the spirits in prison; which sometime were disobedient, when once the longsuffering of God waited in the days of Noah, while the ark was a preparing, wherein few, that is, eight souls were saved by water. The like figure whereunto even baptism doth also now save us (not the putting away of the filth of the flesh, but the answer of a good conscience toward God,) by the resurrection of Jesus Christ: who is gone into heaven, and is on the right hand of God; angels and authorities and powers being made subject unto him.
What surprises me is not that modern evangelicals are unable to reckon with these passages, or that they misinterpret Scripture. Many parts of the Bible have become a puzzle to evangelicals who have long imbibed the ways of the world, so that little else is imaginable. But what does surprise me is that modern evangelicals, who purportedly believe in ‘sola Scriptura’ and sometimes criticize Romanists for following another principle, do not even attempt to justify their understanding of even something so important as the meaning of baptism on the basis of Scripture. Not one verse have I heard cited in support of the understanding that the point – maybe even the only point – of baptism is to show other people that you have made a personal commitment to Jesus.
Baptism is an ordinance of the New Testament, ordained by Jesus Christ, to be unto the party baptized, a sign of his fellowship with him, in his death and resurrection; of his being engrafted into him; of remission of sins; and of giving up into God, through Jesus Christ, to live and walk in newness of life.
The prooftexts listed are: Romans 6.3–5; Colossians 2.12; Galatians 3.27; Mark 1.4; Acts 22.16; Romans 6.4. In this understanding, much like those of other Reformed Protestants, baptism is ‘ordained by Jesus Christ’ as a sign ‘unto the party baptized’; the word is not of that of the believer but that of Christ, who ordained the sign. Modern Protestants, take heed.