Matthew never forgot how, as a tax collector for the Roman Empire, he was disdained as a swindler and traitor to his people. But Jesus called him into service. You do not have to be perfect to serve the Lord. This is often how Jesus works, he calls people who are weak, people who are flawed, people who are the most unlikely candidates. These people–sinners like you and me–he forgives and calls his disciples.
God calls people into his service to share his gospel and to serve others. Through people set apart he does his work. As we proclaim his Word and as we serve humbly the Spirit unleashes his power.