Objection 1. It would seem that Christ is not the Head of all men. For the head has no relation except to the members of its body. Now the unbaptized are nowise members of the Church which is the body of Christ, as it is written (Ephesians 1:23). Therefore Christ is not the Head of all men.
Objection 2. Further, the Apostle writes to the Ephesians (5:25-27): "Christ delivered Himself up for" the Church "that He might present it to Himself a glorious Church, not having spot or wrinkle or any such thing." But there are many of the faithful in whom is found the spot or the wrinkle of sin. Therefore Christ is not the Head of all the faithful.
Objection 3. Further, the sacraments of the Old Law are compared to Christ as the shadow to the body, as is written (Colossians 2:17). But the fathers of the Old Testament in their day served unto these sacraments, according to Hebrews 8:5: "Who serve unto the example and shadow of heavenly things." Hence they did not pertain to Christ's body, and therefore Christ is not the Head of all men.
On the contrary, It is written (1 Timothy 4:10): "Who is the Saviour of all men, especially of the faithful," and (1 John 2:2): "He is the propitiation for our sins, and not for ours only, but also for those of the whole world." Now to save men and to be a propitiation for their sins belongs to Christ as Head. Therefore Christ is the Head of all men.
I answer that, This is the difference between the natural body of man and the Church's mystical body, that the members of the natural body are all together, and the members of the mystical are not all together--neither as regards their natural being, since the body of the Church is made up of the men who have been from the beginning of the world until its end--nor as regards their supernatural being, since, of those who are at any one time, some there are who are without grace, yet will afterwards obtain it, and some have it already. We must therefore consider the members of the mystical body not only as they are in act, but as they are in potentiality. Nevertheless, some are in potentiality who will never be reduced to act, and some are reduced at some time to act; and this according to the triple class, of which the first is by faith, the second by the charity of this life, the third by the fruition of the life to come. Hence we must say that if we take the whole time of the world in general, Christ is the Head of all men, but diversely. For, first and principally, He is the Head of such as are united to Him by glory; secondly, of those who are actually united to Him by charity; thirdly, of those who are actually united to Him by faith; fourthly, of those who are united to Him merely in potentiality, which is not yet reduced to act, yet will be reduced to act according to Divine predestination; fifthly, of those who are united to Him in potentiality, which will never be reduced to act; such are those men existing in the world, who are not predestined, who, however, on their departure from this world, wholly cease to be members of Christ, as being no longer in potentiality to be united to Christ.
Reply to Objection 1. Those who are unbaptized, though not actually in the Church, are in the Church potentially. And this potentiality is rooted in two things--first and principally, in the power of Christ, which is sufficient for the salvation of the whole human race; secondly, in free-will.
Reply to Objection 2. To be "a glorious Church not having spot or wrinkle" is the ultimate end to which we are brought by the Passion of Christ. Hence this will be in heaven, and not on earth, in which "if we say we have no sin, we deceive ourselves," as is written (1 John 1:8). Nevertheless, there are some, viz. mortal, sins from which they are free who are members of Christ by the actual union of charity; but such as are tainted with these sins are not members of Christ actually, but potentially; except, perhaps, imperfectly, by formless faith, which unites to God, relatively but not simply, viz. so that man partake of the life of grace. For, as is written (James 2:20): "Faith without works is dead." Yet such as these receive from Christ a certain vital act, i.e. to believe, as if a lifeless limb were moved by a man to some extent.
Reply to Objection 3. The holy Fathers made use of the legal sacraments, not as realities, but as images and shadows of what was to come. Now it is the same motion to an image as image, and to the reality, as is clear from the Philosopher (De Memor. et Remin. ii). Hence the ancient Fathers, by observing the legal sacraments, were borne to Christ by the same faith and love whereby we also are borne to Him, and hence the ancient Fathers belong to the same Church as we.
Source: Summa Theologica (New Advent)