If Paul was wrong about the occurrence of the Parousia during his lifetime, to what extend does that mistaken view affect reader’s confidence in his teachings?
There are two points that might need to be made here. Initially, I think that with any religious teaching, there is a danger in being very dogmatic about such a complex issue as spiritual identity. Spiritual identity and those who teach or instruct about it are complex realities. Within any complexity, one has to engage in critical reflection. Shortcuts, such as being able to discredit an entire body of thought with one instance, might not be the best path to take. This might be extended into the realm of Paul's teachings. If, indeed, he was wrong about the Second Coming and the condition of Parousia, I am not sure it repudiates all of his teachings. If faith is a condition of absolute belief, then the reader's confidence might not be shaken because with faith, one transcends the notion of weighing out arguments as if a legal metric was sought to be achieved. If faith is present, I am not sure the reader is going to be effected one way or another because calculations are not necessarily a part of religious faith. Paul's teachings are rooted in this faith, something that is not subject to calcuation as much as it is in zeal.
The other issue here might be whether he was wrong, in the first place. Paul's notion of the Parousia was not necessarily limited to his lifetime. Paul stressed that religious faith in Jesus and his teachings are beyond the condition of temporality. Paul was redeemed in his lifetime and this becomes his Parousia that he wishes to impart on others. In this, one could make the argument that Paul might not have been wrong about Parousia. Paul stresses that faith in the redemption of Christ embodies the Parousia, suggesting that it is not a singular moment or instant in which the Second Coming is evident. It is more of a transcendent state of being that the individual and all experience when absorbing the lessons of Christ. The Parousia of Jesus is his "presence" or "arrival" of Jesus in the life of the individual, something that Paul does not limit to a particular moment in time. Testament to the power of Christ, Paul believes that Parousia happens outside of limiting, temporal conditions.
The teachings of Paul as presented by the New Testament have become a major part of Christian dogma and literary cannon. According to the book of Acts and the letters to the Corinthians, Paul believed that the Christ would physically return to the earth during his lifetime. Paul's mistaken view that the Parousia would occur during his lifetime does not affect most modern-day readers' confidence in his teachings.
Christian scholars have reinterpreted the writings attributed to Paul to account for his view. Some scholars suggest that Paul's Parousia is meant to be understood as symbolic. They claim that Paul was speaking of the second coming of Christ that occurs ritualistically during Holy Communion. Others point to the Day of Pentecost as a kind of second coming. Still other theologians suggest that Paul's Parousia did occur, through the visions that Paul himself had of the Christ.
These various interpretations lend a degree of consistency to Paul's teachings, and readers are not likely to have a diminished sense of trust of confidence in Paul's teachings due to his views concerning the Parousia.
Many theologians would say that Paul looked forward to, and believed it to be important to be ready for Christ's bodily coming, but not necessarily during his lifetime. In fact Paul states that the coming of the Lord could not be realized "unless the rebellion comes first, and the man of lawlessness is revealed, the son of destruction, who opposes and exalts himself against every so-called god or object of worship, so that he takes his seat in the temple of God, proclaiming himself to be God." (Second Thessalonians 2:3, 4) Paul is comforting the church at Thessalonica by arguing that Christ had not yet appeared because these two events had not yet been realized. Each generation of Christians is to be watchful and expectant of Christ's Second Coming, but none of us know when that time will come.