In two concluding chapters, Gentry and Wellum summarize their argument and spell out some of its implications. The authors betray (not unfairly) their own ecclesiastical location by stressing that only those who truly believe belong to the new covenant and that the new covenant sign of baptism should therefore be administered only to (professing) believers . . . I think each of these points is well-grounded in the larger argument of the book.
Now, I think something that is important for everyone to understand is that the way that I argue for the legitimacy for infant baptism, and why I think the Bible teaches what is behind it, has nothing to do with the arguments usually leveled against it. In other words, these arguments don't touch my argument. Everyone believes that faith is needed to enter the community of God. You must believe in order to be saved. Period. That's not the issue. The issue is that if parents and children are seen as a single entity, then the head of that entity can make a federal decision to exercise faith for that entity. Hence, the head of the entity exercises faith and the entity is saved by grace through faith. The entity is a single entity exercising faith.
I've tried to say this before, so let me say it again, this is how the gospel itself works. Salvation is gained by works. It's just that no one, but Christ alone, has ever (or could ever) accomplished it. Hence, we, the body of Christ (i.e., one entity), have obtained salvation because our head has obtained it. We are not separate from Christ then. If we are, good luck obtaining salvation for yourself. So the point is that the entity of the church is one with Christ and obtains salvation through Him, and the entity of a family, as long as they are truly in submission to the head, is one, and exercise faith through that federal head so as to be placed within the body of Christ. Ergo, the infant is saved and may be baptized along with the rest of the body that belongs to its federal head, just as we are one and should be made holy along with our federal head, Christ.
I've argued before that the only justification for God killing babies in the OT for their parent's crimes, and the only justification for God saving babies/family members in the OT for their parent's faith, is this principle. Likewise, the only justification for condemning all of mankind through Adam and the only justification for saving all in Christ is this principle. So either the principle is valid, and one now needs to make the argument (not simply give his opinion) that this principle is true for the family of Adam and the family of Christ but somehow not for the individual family, or he needs to admit that one individual who is one entity with his family can exercise faith for that one entity and that single entity then is included in the phrase "through faith."
So faith must be exercised by an individual in order for that one to receive salvation, just as salvation had to be obtained by an individual through works. It's just that the individual is one entity with others, so that whatever he owns is a single entity with him, and therefore, receives his fate.
Whether one agrees or not, that is the argument with which one must grapple, not whether faith is required for entrance into the kingdom. What orthodox Christian would disagree with that?