It is not often thought, among Protestants, that the sacraments or ordinances (baptism and the Lord’s Supper) are necessary. This, however, is understandable, as the necessity of the sacraments is often thought of strictly in the sense or context of the means by which one is saved. Yet, we would be wrong to think of the necessity of the sacraments in this strict manner. With that being said, let me now quote Louis Berkhof on this very issue:
Roman Catholics hold that baptism is absolutely necessary for all unto salvation, and that the sacrament of penance is equally necessary for those who have committed mortal sins after baptism; but that confirmation, the eucharist, and extreme unction are necessary only in the sense that they have been commanded and are eminently helpful. Protestants, on the other hand, teach that the sacraments are not absolutely necessary unto salvation, but are obligatory in view of the divine precept [Hence, Baptists typically refer to the sacraments as ordinances]. Wilful neglect of their use results in spiritual impoverishment and has a destructive tendency, just as all wilful and persistent disobedience to God has. That they are not absolutely necessary unto salvation, follows: (1) from the free spiritual character of the gospel dispensation, in which God does not bind His grace to the use of certain external forms, John 4:21,23; Luke 18:14; (2) from the fact that Scripture mentions only faith as the instrumental condition [or cause] of salvation, John 5:24; 6:29; 3:36; Acts 16:31; (3) from the fact that the sacraments do not originate faith but presuppose it, and are administered where faith is assumed, Acts 2:41 [see also 10:42-48]; 16:14,15,30,33; 1 Cor. 11:23-32; and (4) from the fact that many were actually saved without the use of the sacraments. Think of the believers before the time of Abraham and of the penitent thief on the cross. [Systematic Theology, (1941). 618-619.]
The only thing I would add to Berkhof’s comment, which is certainly implied within, is that baptism is the sign of the New Covenant, and therefore serves as the rite by which one enters into the communion of the Church (e.g. Acts 2:41-42). The same is true of the Lord’s Supper, representing continual communion/fellowship with Christ and His Church (e.g. 1 Cor. 10:16-17; 11:24-26). In other words, those who profess faith in the word of Christ (the gospel), yet refuse baptism and continually neglect the Lord’s Supper, demonstrate a serious inconsistency with their profession, and are actually distancing themselves from, rather than identifying with, Christ and His Church.