“Accordingly, it is said in Mark, He that shall believe and be baptized shall be saved. By these words Christ not only excludes from the hope of salvation hypocrites who, though destitute of faith, are puffed up only by the outward sign; but by a sacred bond he connects baptism with doctrine, so that the latter is nothing more than an appendage of the former. But as Christ enjoins them to teach before baptizing, and desires that none but believers shall be admitted to baptism, it would appear that baptism is not properly administered unless when it is preceded by faith. On this pretence, the Anabaptists have stormed greatly against infant baptism. But the reply is not difficult, if we attend to the reason of the command. Christ orders them to convey to all nations the message of eternal salvation, and confirms it by adding the seal of baptism. Now it was proper that faith in the word should be placed before baptism, since the Gentiles were altogether alienated from God, and had nothing in common with the chosen people; for otherwise it would have been a false figure, which offered forgiveness and the gift of the Spirit to unbelievers, who were not yet members of Christ. But we know that by faith those who were formerly despised are united to the people of God.
It is now asked, on what condition does God adopt as children those who formerly were aliens? It cannot, indeed, be denied that, when he has once received them into his favour, he continues to bestow it on their children and their children’s children. By the coming of Christ God manifested himself as a Father equally to the Gentiles and to the Jews; and, therefore, that promise, which was formerly given to the Jews, must now be in force towards the Gentiles, I will be thy God, and the God of thy seed after thee, (Gen. 17:7.) Thus we see that they who entered by faith into the Church of God are reckoned, along with their posterity, among the members of Christ, and, at the same time, called to the inheritance of salvation. And yet this does not involve the separation of baptism from faith and doctrine; because, though infants are not yet of such an age as to be capable of receiving the grace of God by faith,  still God, when addressing their parents, includes them also. I maintain, therefore, that it is not rash to administer baptism to infants, to which God invites them, when he promises that he will be their God.” 
 My understanding is that God most certainly can and does (though I don’t think we can say dogmatically that he always does) unite infants to Christ. If covenant infants are united to Christ, and I believe they often are, they are united to Christ by faith alone which I would call the “seed of faith” planted in their hearts. Infants cannot have faith as defined by HC 21, however, they can have the “seed of true faith” and its benefit, union with Christ. God works salvation as He pleases, and to say that infants cannot be saved is naive at best, and if they are saved, as I believe at least many are, they are saved by grace alone through faith alone in Christ alone. Again, the “seed of faith” unites them to Christ, though it is not a full-grown faith as HC 21 describes. That is my view.
 John Calvin and William Pringle, Commentary on a Harmony of the Evangelists Matthew, Mark, and Luke, vol. 3 (Bellingham, WA: Logos Bible Software, 2010), 385–386.