Remember that these verses are concerned not with justification and redemption, but with the way in which a preacher carries out his task. When Paul thinks of his own work as a preacher, he can find nothing against himself. Still, the right to judge does not belong to him. Since he is a servant and steward of Christ, it is Christ (the Lord
) who is to judge him.
What Paul says concerning himself as an apostle is true of all office bearers in the church. They too are stewards (Titus 1:7). Although they are chosen by church members and although church members should ensure that they are trustworthy (Titus 1:7), they are ultimately responsible to Christ. They will have to give an account to him (Hebrews 13:17).
Note that Paul uses the present tense: judges.
The Lord is already judging his servants, testing their hearts (1 Thessalonians 2:4).
4 For I am not aware of anything against myself, but I am not thereby acquitted. It is the Lord who judges me.