In an interesting exchange I have had, the case for credobaptism was presented in terms of the redemptive historical paradigm of progressive revelation. In one line of argumentation, there is "this age," and there is "the age to come." The New Covenant belongs to the eschatological age, and since it is eschatological, the New Covenant continues to eternity. Infant baptism in this view belongs to this age, and thus should not be done under the New Covenant, since it does not continue to eternity. The Reformed view, according to this view, must believe that the New Covenant discontinues when Christ comes back, for after all those who enter heaven are those who believe in Christ. However, is such an argument sound?
First of all, it must be said that there are many things done in the New Covenant that will not continue to eternity. The special offices of the church will not be present in heaven, for there all will know the Lord (Jer. 31:34). There will therefore be no preaching, and there will be no Lord's Supper in heaven either. Why will we want to partake of the sign when the reality signified is present? Instead of the Supper, we will partake of the marriage supper of the Lamb, which probably does not consist of a small wafer and a small cup of wine. Therefore, the credobatist faces the same problem. Unless one wants to follow the Quakers, one cannot claim to be "thoroughly eschatological."
Secondly, this argument confuses the substance of the New Covenant with its administration. The substance of the New Covenant is the fullness of the Covenant of Grace, the revelation of the mystery once concealed. All of the Old Testament pointed to Christ and His work (Lk. 24:27), as a shadow to the brightness of the sun. The New Covenant will continue as to its substance, but its form and administration is adapted to the in-between times, the already and the not yet, since we are after all still living in this sinful world. The Eschaton will not usher in an even newer covenant, but rather will consummate the Covenant of Grace in the full redemption of His people.
One cannot claim the "newness" of the New Covenant as the automatic proof for credobaptism. Nobody disputes that the New Covenant is new. The question however is, "What is new about the New Covenant?" Just because the Old Covenant has certain practices does not mean that the New Covenant must necessarily change the practices done.
This line of argumentation therefore is not sound. While I respect my credobaptist brothers, they have to do better than this if they wish to convince us of their position.