Why do people believe that the 144,000 of Rev. 7 will be evangelists?

For instance, in the *Tim LaHaye Prophecy Bible,* in his notes for Rev. 7:1-8: "... we are given an insight into the divine protection of the earth from powerful winds by the assignment of powerful angels and the sealing and **ministry of the 144,000 witnesses.** ... We can only imagine what **evangelistic success these 144,000 servants of God** will have." Where do people get the idea of ministry for the 144,000? Is it stated somewhere in the OT? I don't see any evangelistic work during Revelation's last days by men, only by an angel in Rev. 14:6.