Evangelicals and Protestants are not unified on the answer to this question. Answers would range from "all" to "some" to "none." The Arminian-Calvinist debate weighs on how this question is viewed.
The problem with a Catholic take on this is that it seems to be crossing its fingers behind its back on the promises of scripture. Will you or won't you live forever if you 'eat His flesh and drink His blood'? The Catholic can only say maybe. The Bible says yes.
Here is the passages in question: Jesus said to them, "I tell you the truth, unless you eat the flesh of the Son of Man and drink his blood, you have no life in you. Whoever eats my flesh and drinks my blood has eternal life, and I will raise him up at the last day." (John 6:53-54)
Every promise in Scripture has to be interpreted properly, in context and with the qualifications that are made elsewhere. For example, we are also told that if we believe, we shall be saved. But that does not mean that we can believe, then cease to believe, and still have eternal life. Elsie listed out some verses that deal with that here. Likewise, the fact that eating and drinking His body and blood gives us eternal life does not mean that faith isn't necessary, that perseverance isn't necessary, that baptism isn't necessary, that works are not necessary, etc. The Scripture teaches (in other places) that these are each necessary for salvation, ceteris paribus.
-A8