I quoted that section above from his views on infant baptism. He is stating that if an infant is baptized then he is regenerated. But here is the whole paragraph:
17. They seem to think they produce their strongest reason for denying baptism to children, when they allege, that they are as yet unfit, from nonage, to understand the mystery which is there sealed, viz., spiritual regeneration, which is not applicable to earliest infancy. Hence they infer, that children are only to be regarded as sons of Adam until they have attained an age fit for the reception of the second birth. But all this is directly opposed to the truth of God. For if they are to be accounted sons of Adam, they are left in death, since, in Adam, we can do nothing but die. On the contrary, Christ bids them be brought to him. Why so? Because he is life. Therefore, that he may quicken them, he makes them partners with himself; whereas these men would drive them away from Christ, and adjudge them to death. For if they pretend that infants do not perish when they are accounted the sons of Adam, the error is more than sufficiently confuted by the testimony of Scripture, (1 Cor. 15: 22.) For, seeing it declares that in Adam all die, it follows, that no hope of life remains unless in Christ. Therefore that we may become heirs of life, we must communicate with him. Again, seeing it is elsewhere written that we are all by nature the children of wrath, (Eph. 2: 3,) and conceived in sin, (Ps. 51: 5,) of which condemnation is the inseparable attendant, we must part with our own nature before we have any access to the kingdom of God. And what can be clearer than the expression, "Flesh and blood cannot inherit the kingdom of God?" (1 Cor. 15: 50.) Therefore, let every thing that is our own be abolished, (this cannot be without regeneration,) and then we shall perceive this possession of the kingdom. In fine, if Christ speaks truly when he declares that he is life, we must necessarily be ingrafted into him by whom we are delivered from the of death. But how, they ask, are infants regenerated, when not possessing a knowledge of either good or evil? We answer, that the work of God, though beyond the reach of our capacity, is not therefore null. Moreover, infants who are to be saved (and that some are saved at this age is certain) must, without question, be previously regenerated by the Lord. For if they bring innate corruption with them from their mother's womb, they must be purified before they can be admitted into the kingdom of God, into which shall not enter any thing that defileth, (Rev. 21: 27.) If they are born sinners, as David and Paul affirm, they must either remain unaccepted and hated by God, or be justified. And why do we ask more, when the Judge himself publicly declares, that "except a man be born again, he cannot see the kingdom of God?" (John 3: 3.) But to silence this class of objectors, God gave, in the case of John the Baptist, whom he sanctified from his mother's womb, (Luke 1: 15,) a proof of what he might do in others. They gain nothing by the quibble to which they here resort, viz., that this was only once done, and, therefore, it does not forthwith follow that the Lord always acts thus with infants. That is not the mode in which we reason. Our only object is to show, that they unjustly and malignantly confine the power of God within limits, within which it cannot be confined. As little weight is due to another subterfuge. They allege that, by the usual phraseology of Scriptures "from the womb," has the same meaning as "from childhood." But it is easy to see that the angel had a different meaning when he announced to Zacharias that the child not yet born would be filled with the Holy Spirit. Instead of attempting to give a law to God, let us hold that he sanctifies whom he pleases in the way in which he sanctified John, seeing that his power is not impaired.