By a sovereign decree, independently of any human point of view, God decides that the children of believers shall be included in His covenant. In His sovereignty He imposes this relationship upon them. It pleases God to make this covenant with them. He chooses these children as heirs of the promise. God’s decision and the offer of the blessings of the covenant precede the faith of the child. The character of this covenant is sovereignly objective.
The children of believers are the heirs of all the promises of the covenant. In the same way as the baptized proselyte they are separate from the profane world and are placed neither under God’s judgment nor under Satan’s power. God regards them as members of His kingdom. He promises to circumcise their hearts in order that they may love Him and live. He wishes to be their Father, to cause them to enjoy the benefits of His grace, and to lead them to salvation. The children of believers are considered by God as being involved in the faith of their parents; the family, as such, forms a concrete whole. They are members of the Church.
Since the children of believers are “set apart” separate from the profane world, “holy” -to use the Biblical expression-from the moment of their birth; since God includes them in the covenant and they belong to Christ’s body the Church; in short, since they participate in all the promises and all the spiritual realities signified and sealed in baptism, we say that they are fit to receive it; there is no other reason for administering baptism to them. The grace of their adoption precedes baptism which is the sign and seal of it. This basis of the baptism of children is fundamentally objective and rigorously established upon the New Testament.
The covenant of grace, in fact, remains one and the same in the New Testament, the Church remains the same, and the children of believers are part of it. Christ confirms the spiritual solidarity of the family. In the New Testament we encounter the same effects of the covenant of grace relating to the children of believers. If the effects are the same, so also are the principles. (Pierre Marcel in The Doctrine of Infant Baptism)