An apostle is a delegate, envoy, agent, ambassador or representative, indicating one sent with a message or commission. Jesus Himself is called an apostle in Hebrews 3:1, and He appointed twelve to spread the gospel after Him. Other men are designated apostles in the New Testament: Paul, Barnabas, James and Jude are definitely in this class, and Paul calls the unknown Andronicus and Junia "apostles" in Romans 16:7, as he also seems to do with Silas and possibly Timothy in I Thessalonians 2:6.
Apostles of Christ show "signs of an apostle" (II Corinthians 12:12), which are the fruits of their ministry, such as preaching the gospel, healings, demons being cast out, people being converted, congregations being raised up and clarifying doctrine. Herbert Armstrong fulfilled all of these. As our "father in the faith," directly or indirectly, he played a part in each of our conversions.
However, like Paul, he was an apostle "out of due time" (I Corinthians 15:8). In no way was he equal to the original Twelve, who will rule over the twelve tribes of Israel in the Kingdom (Luke 22:30; Revelation 21:12-14). Herbert Armstrong was raised by God as an apostle to fulfill a commission to preach the gospel and raise up the church as the end approached.
As for being the "Elijah to come," Herbert Armstrong may have fulfilled some of Elijah's roles (Malachi 3:1-3; 4:5-6), but as time goes on, it seems he did not completely fulfill the type. In fact, Jesus in Matthew 17:10-13 shows that John the Baptist fulfilled the type. If this type is dual, another "Elijah" will arise before Christ's second coming to prepare the way for His return and the establishment of the Kingdom of God.