I answer that, Baptism has the power to take away the penalties of the present life yet it does not take them away during the present life, but by its power they will be taken away from the just in the resurrection when "this mortal hath put on immortality" (1 Cor. 15:54). And this is reasonable. First, because, by Baptism, man is incorporated in Christ, and is made His member, as stated above (Article [3]; Question [68], Article [5]). Consequently it is fitting that what takes place in the Head should take place also in the member incorporated. Now, from the very beginning of His conception Christ was "full of grace and truth," yet He had a passible body, which through His Passion and death was raised up to a life of glory. Wherefore a Christian receives grace in Baptism, as to his soul; but he retains a passible body, so that he may suffer for Christ therein: yet at length he will be raised up to a life of impassibility. Hence the Apostle says (Rm. 8:11): "He that raised up Jesus Christ from the dead, shall quicken also our [Vulg.: 'your'] mortal bodies, because of His Spirit that dwelleth in us [Vulg.: 'you']": and further on in the same chapter (Rm. 8:17): "Heirs indeed of God, and joint heirs with Christ: yet so, if we suffer with Him, that we may be also glorified with Him."
Secondly, this is suitable for our spiritual training: namely, in order that, by fighting against concupiscence and other defects to which he is subject, man may receive the crown of victory. Wherefore on Rm. 6:6, "that the body of sin may be destroyed," a gloss says: "If a man after Baptism live in the flesh, he has concupiscence to fight against, and to conquer by God's help." In sign of which it is written (Judges 3:1,2): "These are the nations which the Lord left, that by them He might instruct Israel . . . that afterwards their children might learn to fight with their enemies, and to be trained up to war."
Thirdly, this was suitable, lest men might seek to be baptized for the sake of impassibility in the present life, and not for the sake of the glory of life eternal. Wherefore the Apostle says (1 Cor. 15:19): "If in this life only we have hope in Christ, we are of all men most miserable."