Who are the 144,000 in Revelation 7 and 14? Do they refer to Jewish evangelists who live during the seven years of Tribulation as a popular commentary that I consulted suggests? 

No, I do not think that they refer to Jewish evangelists. In fact, I do not even think that there will be seven years of Great Tribulation, or that the book of Revelation is entirely about future events, though it does touch on some aspects of the future (see NTIAGQ in PCC Bulletin, vol. 1, no. 45; dated 7 May 2000 for an introduction to the different approaches to interpreting the book of Revelation).

I am convinced that the 144,000 refer to the true “Israel of God” (Gal 6:16; cf. Jas 1:1), which comprises all the elect of Christ throughout the ages. My reasons in brief are as follows

First, the number 144,000 or 12 times 12,000 is clearly a symbolic number. Notice how the twelve tribes listed (Rev 7:5-8) exclude Dan, but include Levi and Joseph. This suggests that we are not to understand the twelve tribes as referring literally to the twelve tribes in Old Testament history, for in the Old Testament censes, Dan is always counted (e.g. Num 1:38, 16:42), whereas Levi is usually excluded (see Num 1:47) or counted separately (see Num 16:51, 57ff); and Ephraim is never called the Joseph, though Manasseh is once named as Joseph (Num 13:11; Num 1:32 speaks of Joseph as comprising Ephraim and Manasseh, v. 34). Some say that Dan was excluded because of apostasy or because the antichrist is expected to arise from her but there is no biblical basis for such a view (see O Palmer Robertson, The Israel of God [P&R, 2000], 156 fn).

Secondly, the 144,000 are said to be “the servants of our God” (Rev 7:3) and those “which were redeemed from the earth… which follow the Lamb whithersoever he goeth” (Rev 14:3-4). Who are these but Christians “redeemed… to God by [the] blood [of the Lamb] out of every kindred, and tongue, and people, and nation” (Rev 5:9; cf. 1 Pet 1:18). 

Thirdly, the 144,000 is quite clearly connected to the “great multitude, which no man could number, of all nations, and kindreds, and people, and tongues, [standing] before the throne, and before the Lamb, clothed with white robes…” (Rev 7:9). John heard that 144,000 would be sealed (v. 4), but after he heard the listing of them, he beheld and saw the great multitude from every nation (v. 9). Those who object to this connection by saying that John heard about a group of people but saw another group, should realize this is not the first time in this book, that something is described to John in some terms, but when he sees it personally, he describes it differently. In particular, in chapter 5, one of the elders charges him to behold “the Lion of the tribe of Judah” (v. 5), but when he turned to look, he saw instead, “a Lamb as it had been slain” (v. 6).

I believe these three reasons are conclusive in identifying the 144,000 with the redeemed throughout the ages. But why 144,000? Well, whatever else may be said about the number, it is clear that it is derived from the number 12, which I believe points to the church as the covenant body of Christ. This is why the Scripture speaks about twelve tribes when there are really thirteen tribes in the Old Testament if we consider Ephraim and Manasseh as one tribe each (as done in Rev 7:5-8). This is also why the New Testament speaks about 12 apostles (being seed members of the New Testament church) even though, there are really thirteen if we include Matthias and Paul (excluding Judas and Barnabas). This is why when the Holy Spirit would have John know that the “bride of the Lamb” (Rev 21:9) comprises the church spanning the Old and New Testament, a symbolic reference is made to the twelve tribes of Israel and the twelve apostles (see Rev 21:12 and 14). 

In short, whatever else may be said about the 144,000, we can be sure that it speaks of the redeemed from “all nations, and kindreds, and people, and tongues” (v. 9) as being united in one covenant body or one church,—namely the universal invisible church of Christ.