§ 8. The Necessity of the Sacraments

Necessity is what cannot not be (S. th. I 82, I : Necesse est quod non potest non esse), whether on the ground of an inner principle (for example the Existence of God), or on the ground of an external principle. The latter can be a final or an efficient cause. The necessity imposed by the final cause (necessitas finis) consists in this that unless one uses a defined means one cannot achieve a certain purpose (for example, nourishment is necessary (necessitate finis) for the preservation of human life) or at least one cannot achieve it easily (for example, a vehicle is necessary for the easy performance of a long journey). The necessity imposed by an efficient cause is the necessity of coaction (necessitas coactionis). A high grade of appropriateness is called necessity of congruity (necessitas convenientiae or congruentiae). S. th. I 82, 1.

1. On the Part of God
God can communicate grace even without the Sacraments. (Sent. certa.)
God, in His omnipotence and freedom, can communicate grace in a purely spiritual manner also. Therefore for Him the institution of Sacraments was not indispensably necessary. S. th. Ill 72, 6 ad 1 ; virtus divina non est alligata sacramentis. However, in view of the fact that man is composed of body and soul the institution was highly appropriate (necessitas convenientiae or congruentiae). Again, the nature of the Church as a visible religious community demands visible religious signs (notae quaedam et symbola quidus fideles intemoscerentur ; Cat. Rom. II 1, 9, 4). The Sacraments are also valuable in promoting the Christian life of virtue : humility, on account of man’s dependence for his spiritual life on material elements ; faith and confidence, on account of the manifest signs of grace ; neighbourly love, on account of the co-membership of one Mystical Body implied by the Sacraments. Cf. S. th. Ill 61, 1 ; S.c.G. IV 56 ; Cat. Rom. II 1, 9.

2. On the Part of Man
The Sacraments of the New Covenant are necessary for the salvation of mankind. (De fide.)
As Christ instituted the Sacraments and bound them up with the communication of grace they are necessary to us for the achievement of salvation (necessitate medii), even if not all are necessary for each individual. The efficacious reception of a Sacrament can, in case of necessity, be replaced by the desire for the Sacrament (votum sacramenti) (hypothetical necessity).
The Council of Trent declared against the Reformers who, on the ground of their “sola fides” doctrine, contested the necessity of the Sacraments for salvation : Si quis dixerit sacramenta novae Legis non esse ad salutem necessaria, sed superflua, et sine eis aut eorum voto per solam fidem homines a Deo gratiam iustificationis adipisci, licet omnia singulis necessaria non sint. A.S. D. 847. In the Middle Ages the necessity of the Sacraments was controverted by the Cathari.
The Sacraments are the means appointed by God for the attainment of eternal salvation. Three of them are in the ordinary way of salvation so necessary. that without their use salvation cannot be attained. Thus, for the individual person, Baptism is necessary in this way and after the commission of a grievous sin, Penance is equally necessary, while for the Church in general, the Sacrament of Holy Orders is necessary. The other Sacraments are necessary in so far as salvation cannot be so easily gained without them. Thus Confirmation is the completion of Baptism, and Extreme Unction is the completion of Penance, while Matrimony is the basis for the preservation of the Church commonwealth, and the Eucharist is the end (finis) of all the Sacraments. Cf. S. th. Ill 6j, 3 and 4.

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Ref: Ludwig Ott. Fundamentals of Catholic Dogma. Ed. James Bastible, trans. Patrick Lynch. (Rockford, IL: Tan Books, 1974). p.340.
Imprimatur: Cornelius, Ep.Corgagiensis at Ap. Adm, Rossensis. 7 October 1954.

+AMDG+
Feast of St. Joseph

I was quite surprised when reading the catechism that the Sacrament of Baptism and Penance were the two absolute necessaries for salvation.

Interestingly, coming from the NO you grow into an understanding that Holy Communion is what get's you into Heaven, maybe that is why there is an obsession to partake in receiving the sacrament even though some do not believe, are indifferent, have not gone to confession since their first confession, etc.
There is the nagging belief that they ought to receive no matter what!  Even unworthily!

Then there is the other extreme where Traditionalists feel that their entry into Heaven absolutely hinges on last rites, to the point of mania!
They live their lives obsessed with the thought of being in Fr. SSPX's graces fearing that if you do not get the Sacrament, you won't get to Heaven

Both uncannily then lead to the grave error of the infallibility of the priest, where everything Fr. SSPX says has to be obeyed, even if it has nothing to do with Faith and morals, and even when it goes against the Faith.  Priest worship.

Many saints died without receiving last rites.
Food for thought.

Three sacraments are necessary for salvation. Two of them are necessary to the individual; Baptism, simply and absolutely; Penance, in the case of mortal sin committed after Baptism; while the sacrament of Holy Orders is necessary to the Church, since "where there is no governor the people shall fall".