A Christian is indeed justified by grace through faith alone (Rom 1:17; 3:24–28; Gal 2:16). This means that nothing that a man does will contribute to his salvation (cf. Phil 3:7–8). In that sense (and that sense only), you are quite right to say that the sacraments are not absolutely necessary for our salvation. Thus, it is possible for a person who is prevented from the opportunity of baptism and the Lord’s Supper to be saved without the two ordinances. The penitent thief on the cross was not baptised, but he was certainly saved. Our Confession of Faith affirms this doctrine in WCF 28.5—

Although it be a great sin to contemn or neglect this ordinance, yet grace and salvation are not so inseparably annexed unto it, as that no person can be regenerated or saved without it, or that all that are baptised are undoubtedly regenerated.

However, having said thus, we must remember, in the first place, that whoever Christ saves does not remain in sin (1 Jn 3:6), and sin is lawlessness (1 Jn 3:4). The child of God is not only given a new standing; he is given a new nature by way of regeneration (Eph 2:1–5). With the new nature, the child of God will not think that it is optional to obey the commandments of Christ! If Christ has commanded him to be baptised and to partake of the Lord’s Supper, then he knows that “it isn’t enough” for him until he is baptised and is partaking of the Lord’s Supper. Negatively, he will know that it is a “great sin to contemn [i.e., despise] or neglect” these ordinances. And positively, the Holy Spirit who indwells him will so work within his heart that he will desire to pursue after holiness and to seek to please God,—which includes obeying whatever He commands. This is why the Apostle John tells us: “This is the love of God, that we keep his commandments: and his commandments are not grievous” (1 Jn 5:3). One can hardly be a true Christian who has no desire for baptism, or participation at the Lord’s Table.

And furthermore, baptism and the Lord’s Supper are no ordinary ordinances. They are signs and seals of the gracious covenant of God (WCF 27.1). Because these two ordinances share some unique characteristics which other ordinances (e.g., preaching of the Word and prayer) do not share, they are known separately as sacraments. The Westminster Shorter Catechism succinctly defines a sacrament as “an holy ordinance instituted by Christ; wherein,—by sensible signs,—Christ, and the benefits of the New Covenant,—are represented, sealed, and applied to believers” (WSC 92). Notice the word “applied.” We cannot in this space prove that WSC 92 is correct, but we believe it is. This is why the Scriptures speak of baptism as being “for the remission of sins” (Acts 2:38), and regeneration as being the “washing of regeneration” (Tit 3:5). And similarly, the Lord’s Supper is spoken of as being a “communion of the blood [and body] of Christ” (1 Cor 10:16); and the Lord Himself speaks of receiving Him as eating His flesh and drinking His blood (Jn 6:53).

Based on this doctrine, our Confession of Faith teaches us that though “the efficacy of baptism is not tied to that moment of time wherein it is administered; yet, notwithstanding, by the right use of this ordinance, the grace promised is not only offered, but reallyexhibited and conferred by the Holy Ghost,…” (WCF 28.6; emphasis mine). Likewise, for the Lord’s Supper, our Confession affirms: “Worthy receivers, outwardly partaking of the visible elements, in this sacrament, do then also inwardly by faith, really and indeed, yet not carnally and corporally, but spiritually, receive and feed upon Christ crucified, and all benefits of His death…” (WCF 29.7; emphasis mine).

What do we say to this verity, but that the sacraments are needful for our salvation, and under normal circumstance must be made used of by every child of God. This is why our catechism teaches us that apart from faith in Jesus Christ and repentance unto life, God requires of us,—in order that we may escape His wrath and curse due us,—to make “diligent use of all the outward means whereby Christ communicateth to us the benefits of redemption” (WSC 85; emphasis mine). The sacraments,—being specially instituted outward means,—are at least as important as the Word and prayer! We must not think otherwise.

Indeed, we may further infer that as the sacraments are outward and ordinary means to apply the benefits of redemption unto us, it would be seriously doubtful if any true believer will persist on in his Christian life without having any desire to make use of the sacraments. Or shall we say, if a professing Christian,—who, for some less than valid reasons, refuses baptism,—were to be called to the bar of judgement before he actually decides to seek baptism, it would be very likely that he was never one of Christ’s. And likewise, a baptised professing believer, who persists in not taking the Lord’s Supper for one reason or another, would be a spiritually under-nourished believer, if he were a believer at all. The Word and prayer are indeed appointed means of grace, but the sacraments are specially appointed as means to apply the benefits of the covenant unto believers; how can they be neglected and there be no real effect on the soul, whether perceived by the individual or not?

There is a third reason, moreover, as to why baptism and the Lord’s Supper are mandatory for every believer, namely, that the Scripture acknowledges no private believer. Every believer must be a member of the church of Christ and enjoy the communion of saints through union with Christ. The sacraments are the primary appointed outward means by which the communion of saints is manifest and enjoyed. Baptism, apart from its significance already stated, is also a means whereby a believer is engrafted into the covenant community (Acts 2:41; Gen 17:13–14); and the Lord’s Supper, being a divinely appointed covenantal meal that believers share together, is one of the most important spiritual exercise of communion with one another in the church. “For we being many are one bread, and one body: for we are all partakers of that one bread” (1 Cor 10:17), says the Apostle Paul.

With these compelling reasons, we must say that baptism and the Lord’s Supper are not optional. The true child of God will have no rest in his soul until he finds himself a partaker of these means which his Lord and King has so graciously provided. Apart from a desire to demonstrate submission to Christ rather than men, these are some of the reasons why Christians have been known to risk being ostracised by family and society, by being baptised. These are perhaps also the reasons why during the time of the Reformation, Christians were known to risk persecution to enjoy partaking the Lord’s Supper with likeminded saints.