If we are not careful we can distort the difference between the efficacy of baptism, the proper subjects of baptism, and the true type of baptism. There is a real danger in confusing these three things even if the consequences are unintended.
Charles Hodge wrote that the efficacy of the sacraments "does not depend upon any virtue in them or in him by whom they are administered but upon the attending influence of the Holy Spirit." The sacraments, and here baptism particularly, communicates the Triune work of God in our salvation. It does not perform or activate the work of regeneration or justification.
When we confuse the efficacy of baptism, the proper subjects, and the proper and true type of baptism, we give credence to baptismal regeneration or some outward act of man contributing to salvation.
So, I believe first and foremost, we must make a distinction in the categories. If efficacy is tied in any way to the time and mode of baptism, there is no way to avoid, to some degree, the inclusion of outward or physical actions of man to efficacy. By efficacy, we mean the "power to produce effects."
Responding to the question, "How do Baptism and the Lord's Supper become effectual means of salvation?", Benjamin Keach wrote:
"Baptism and the Lord's Supper become effectual means of salvation, not from any virtue in them or in him that administers them, but only by the blessing of Christ and the working of His Spirit in them that by faith receive them."
"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."
Without question, the time and mode of baptism are important, but they are not as important as the nature and efficacy of baptism. The nature of baptism is in what it signifies. The efficacy of baptism is the power to produce what it signifies.
Credobaptists need to be careful not to confuse the efficacy of baptism with any verbal or physical action. I realize baptism possesses important verbal and outward actions, but the proper administration of baptism is a different category than the efficacy of baptism.
When we are not careful, we distort regeneration, which is one of the things baptism is supposed to signify. The point is that although baptism signifies regeneration, it is not regeneration.
Baptism signifies to the party baptized "the covenant of grace, of his ingrafting into Christ, of regeneration, of remission of sins, and of his giving up unto God, through Jesus Christ, to walk in newness of life." What baptism is signifying is God's grace and not our response as an effectual means to this grace. Baptism is a response to grace and not a channel to it. Certainly, God's grace is not void of faith and works, but the implication in baptism is that faith and works proceed from God's action rather than God's action proceeding from our faith and works.
This, I believe, is why our Reformed credobaptist and paedobaptist differences should be understood as timing and mode. If the timing is efficacious it changes the nature of baptism.
We must be careful not to make baptism signify our actions or effectual by our actions. Baptism signifies God's action in giving and our receiving the promises as articulated in the Word. Although there is an outward and inward part to the sacraments, we must maintain that the outward is a sign to the party baptized of God's grace and the inward and spiritual grace is that which is signified in baptism (WLC 163).