Objection 1. It seems that sacraments are not necessary for man's salvation. For the Apostle says (1 Timothy 4:8): "Bodily exercise is profitable to little." But the use of sacraments pertains to bodily exercise; because sacraments are perfected in the signification of sensible things and words, as stated above (Question 60, Article 6). Therefore sacraments are not necessary for the salvation of man.
Objection 2. Further, the Apostle was told (2 Corinthians 12:9): "My grace is sufficient for thee." But it would not suffice if sacraments were necessary for salvation. Therefore sacraments are not necessary for man's salvation.
Objection 3. Further, given a sufficient cause, nothing more seems to be required for the effect. But Christ's Passion is the sufficient cause of our salvation; for the Apostle says (Romans 5:10): "If, when we were enemies, we were reconciled to God by the death of His Son: much more, being reconciled, shall we be saved by His life." Therefore sacraments are not necessary for man's salvation.
On the contrary, Augustine says (Contra Faust. xix): "It is impossible to keep men together in one religious denomination, whether true or false, except they be united by means of visible signs or sacraments." But it is necessary for salvation that men be united together in the name of the one true religion. Therefore sacraments are necessary for man's salvation.
I answer that, Sacraments are necessary unto man's salvation for three reasons. The first is taken from the condition of human nature which is such that it has to be led by things corporeal and sensible to things spiritual and intelligible. Now it belongs to Divine providence to provide for each one according as its condition requires. Divine wisdom, therefore, fittingly provides man with means of salvation, in the shape of corporeal and sensible signs that are called sacraments.
The second reason is taken from the state of man who in sinning subjected himself by his affections to corporeal things. Now the healing remedy should be given to a man so as to reach the part affected by disease. Consequently it was fitting that God should provide man with a spiritual medicine by means of certain corporeal signs; for if man were offered spiritual things without a veil, his mind being taken up with the material world would be unable to apply itself to them.
The third reason is taken from the fact that man is prone to direct his activity chiefly towards material things. Lest, therefore, it should be too hard for man to be drawn away entirely from bodily actions, bodily exercise was offered to him in the sacraments, by which he might be trained to avoid superstitious practices, consisting in the worship of demons, and all manner of harmful action, consisting in sinful deeds.
It follows, therefore, that through the institution of the sacraments man, consistently with his nature, is instructed through sensible things; he is humbled, through confessing that he is subject to corporeal things, seeing that he receives assistance through them: and he is even preserved from bodily hurt, by the healthy exercise of the sacraments.
Reply to Objection 1. Bodily exercise, as such, is not very profitable: but exercise taken in the use of the sacraments is not merely bodily, but to a certain extent spiritual, viz. in its signification and in its causality.
Reply to Objection 2. God's grace is a sufficient cause of man's salvation. But God gives grace to man in a way which is suitable to him. Hence it is that man needs the sacraments that he may obtain grace.
Reply to Objection 3. Christ's Passion is a sufficient cause of man's salvation. But it does not follow that the sacraments are not also necessary for that purpose: because they obtain their effect through the power of Christ's Passion; and Christ's Passion is, so to say, applied to man through the sacraments according to the Apostle (Romans 6:3): "All we who are baptized in Christ Jesus, are baptized in His death."
Source: Summa Theologica (New Advent)