It is interesting to me that Matthew, who lists himself eighth among the twelve apostles, adds a qualifier to his name different from any notation he makes about the others (10:1-4). He identifies one as a traitor, gives an ethnic detail about another, tells us that there are two pairs of brothers chosen by our Lord, gives surnames, nicknames, and tells us James is the son of Alphaeus. Yet, only after his own name does he specify occupation. We know there are fishermen in the group. At least one professional nationalist stood among the chosen. Perhaps one was in the finance business. Strangely, however, he mentions only his trade.
By Matthew's own account, tax collectors had no qualms hanging out with sinners (9:10-11). Luke records at least one known for unscrupulous, unethical behavior in that occupation (19:1ff; cf. 3:13). In fact, Matthew repeatedly lumps together tax collectors, prostitutes, and pagans (11:19; 18:17; etc.). That is apparently how they were seen, especially by the scribes and Pharisees. Yet, Matthew does not hide the kind of work he did before Jesus called him. He openly lists it as the thing that distinguished him.
Did he do this to give hope to people who wrestled with the guilt of their own sins, who would know what he overcame to follow Jesus and see him as an example of Jesus' power? Did he do this to show that even he, who picked to be an apostle and to write one of the four sacred records of Jesus' life, had a sinful past (cf. Rom. 3:23)? Did he do this to show the unity possible in Christ? How much interaction did he and Simon the zealot have in fulfilling Jesus' ministry? Did they ever "door knock" together? How far apart were they when reclining to eat together? A Jewish nationalist would not have had deep, inherent appreciation for a guy collecting taxes to enrich the Roman coffers.
What is clear is that Matthew tells everyone what he did and who he was. Yet, what he became through Christ is what is most important. He is still faithfully serving after Christ's resurrection and ascension (Acts 1:13). He had a role in doing the initial preaching of the gospel of Christ (Acts 2:14).
What about Joe the alcoholic? Or Ted the foul-mouthed? Or Susie the liar? If they repent and obey Jesus, can they find a useful place in the kingdom, too? We already know the answer. The Lord wants us to believe it!