Although the details may vary among various Evangelical Bible groups some facts we can garner about their views on the 144k are:
1 They are not in existence at the present. Their presence is mentioned in Rev ch 7, which begins with the words: "After these things" So, evidently it is only after some events, which are mentioned in Ch 6, that the 144k emerge. [Interestingly, in the Rev book, which gives an exhaustive discussion of this section, in order to suggest the on-going existence of the 144k since 33AD, these three words are artfully ignored. When, in conversation with various WT followers I mentioned these words, requiring a justification for why these 144k can possibly exist "before these things" the reply I was given was that Rev does not always mean what it says. "After these things" can be a symbol for "before these things" With such elastic logic, one can only suggest that the Bible can be made to say any thing]
2 Most agree that Ch 6 tells of the opening of the Great Tribulation, which even the Rev book agrees with.
3 So, somtime after the start of the Great Tribulation, whoever these 144k are, they will begin what is called a "Great Tribulation ministry" ie a preaching work world-wide where the message of the Kingdom of Christ will be preached to all. This and not a current flawed work by the WTS, is the fulfillment of Matt 24:14, which entire Ch is devoted to the future events of the GT
4 The details are not agreed on by all, but these 144k, are indeed a literal number, and literal Jews, evidently young unmarried men, from the various tribes mentioned. It needs to be remembered that Protestant Evangelical groups who still advocate a future blessing for the Jewish people, do not believe, like the WTS believes, that the 12 tribes have become lost. It is just that the means of identifying them are. An archaeological find, an exposure of documents brought about by some earthquake, a discovery in some long lost library etc, could easily provide a means to identifying these tribes.
5 Some Evangelical groups believe that when the GT begins, the Rapture takes place, removing all True Christians from the scene. [Pre-Tribulationists] This will require the work of "born again" Jews to do the preaching work during this time. Some believe that during the 7 year period that marks the GT, the Rapture only occurs at the mid-point, ie after three and a half years, so these 144k Jews will assist the Christans in the preaching work till the rapture, when they will continue on their own.[Mid-Tribulationists]
6 Whatever the details, as a consequence of this evalgelizing work, a number of "Tribulation Saints" will be saved, who will end up "before the throne" as a great crowd. They will be the ones who cared for Christ's "brothers" [the 144k] during the GT [Matt 25:40], and who will gain the approval of Christ for doing so.
These are the views that are held by those who prefer a literalist, futurist interpretation of Rev. Other Evangelicals who hold to either a preterite or historical interpretation, [sometimes referred to as Amillennialists] feel that the 144k "Jews" are a symbolic number representing several things. The most common view is that they represent the Church down through the centuries.
Real Christianity allows for a variety of interpretations, all compatible with the understanding of what the Holy Spirit leads one to believe. Unlike the haughty, pompous, attitude of the WTS whch insists on its own imperatives, I have yet to see one Evangelical "disfellowship" another simply for holding a different opinion.
Cheers