Roman Catholics and Anglicans have historically used the phrase "outward sign of an inward grace" to describe the sacraments, but by this expression, Roman Catholics certainly have not wished to suggest that the sacraments are "only" outward forms that have no direct connection to the inward grace that they signify. Most Protestants only accept two sacraments: baptism, and communion (some, such as the Salvation Army, reject all sacraments -- including baptism), and many of those Protestants believe that these sacraments are "only" outward expressions. They believe that we are saved by "faith alone" and that baptism is merely a public profession of that faith, and the first act of obedience of a Christian... but they do not believe that baptism is necessary for salvation.
As a consequence, it is not uncommon in some Protestant denominations to find people who have never been baptized, and yet consider themselves Christians. I know that when I was attending Southern Nazarene University, in my New Testament Theology class, the professor made a point of asking if anyone in the class had not been baptized, when he got to the point in the course in which Baptism was discussed... and invariably, there would be one or two students who were studying for the ministry, and yet had never been baptized. He would tell them to see him after class, so that he could make the arrangements for them to be baptized.
One does not find such a laissez faire view of Baptism in the New Testament. St. Peter speaks of Ark of Noah, and how it saved Noah and his family as a type (foreshadowing) of Baptism: "There is also an antitype [that which is forshadowed by the type] which now saves us—baptism (not the removal of the filth of the flesh, but the answer of a good conscience toward God), through the resurrection of Jesus Christ" (1 Peter 3:21). This text, read in its context (3:18-22) states that just as the Ark of Noah saved those who were in it, so does Baptism now save us.
The reason why many Protestant traditions took the position that Baptism was only an outward profession of faith is in reaction to the view of many careless Christians who falsely believed that they would be saved simply by virtue of their having been baptized. And such an erroneous view does need to be corrected, but not by denying the clear teachings of Scripture, and the unbroken Tradition of the Church on the matter. Baptism is the portal into the Ark of the Church. However, if someone does not remain in the Ark, they will not benefit by having entered that portal, but will in fact face the greater judgment.
You could think of being baptized as being analogous to having health insurance. When you have health insurance, you have the right to go and see your doctor and receive the treatment that you need. However, if someone has health insurance, but never uses it, that health insurance does them absolutely no good. Likewise, being baptized brings you into the Church, where you have all the sacraments and instruction in righteousness that the Church has to offer, but if you are baptized, but separate yourself from the Church by not bothering to avail yourself of the grace that if to be found there, your baptism has done you no good.
Now, some may ask, does this mean that if someone is not baptized, they cannot be saved? No. One can cite exceptional examples such as the wise thief on the Cross who "stole paradise" "in single moment," or to martyrs who embraced the Christian Faith and were killed before they could be baptized. However, the Scriptures and the Tradition of the Church make it clear that baptism is the normal means by which we should enter the Church, and ultimately enter paradise.
It should be pointed out that there are some Protestants, such as Lutherans and members of the "Campbellite" Church of Christ that do affirm that Baptism actually plays a necessary role in our salvation, and so is not something that one could ignore, and still be considered a Christian.
