"In that for which they slander you as evildoers": standing alone amid a hostile world with their high principles of new life according to the Gospel, Christians were slandered by Gentiles as evildoers because of these very principles.
The Gentiles did not understand these high principles. They misinterpreted and distorted them. Christians suffered from them not only slander, but even persecutions.
Thus, to the superficial and corrupt view of the Gentiles, the brotherly love of Christians, by which they stood in close mutual communion, gave grounds for suspecting them of a secret anti-social union or conspiracy dangerous to the state.
Their firmness in their faith, and their unshakable confession of it amid slander, persecution, and oppression, seemed to the Gentiles a dangerous fanatical stubbornness.
Their estrangement from the Gentiles was interpreted as hatred of mankind. Their break with ancient pagan customs, traditions, and habits was interpreted as contempt for every human institution, and so forth (cf. 1 Pet 2:19-20; 1 Pet 3:10-12, 17; 1 Pet 4:15).
According to the Apostle, only the virtuous life of Christians, or in general their good works, can dispel such views of Christians among the Gentiles.
Not only can it dispel them, but it can even turn the slanderers themselves from slander to glorifying the God confessed by Christians.
Good works, or a virtuous life in general, truly act more strongly upon unbelievers, as experience shows, than teaching does.
"That they may glorify God": the goal of a virtuous life in relation to unbelievers and slanderers is not that people should exalt the virtuous for their virtue, but that God should be glorified (cf. Mt 5:16).
"In the day of visitation": God's visitation is a special action of God toward men. It may be beneficent and merciful (Gen 21:1; Gen 50:24; Job 7:18; Lk 1:68, 78; Acts 15:14), or chastising and punitive (Jer 9:24-25; Jer 44:13; Ps 58:6; Ex 20:5).
Some interpreters understand God's visitation here in the first sense, others in the latter.
Since the Apostle himself does not define this word more closely, one may suppose that he means God's visitation in general. That is, he uses the word in both senses.
The phrase "day of visitation" indicates the time of such visitation in general, since the word "day" signifies not only a day in the proper sense, but time generally.
What time of visitation is meant here? And in relation to whom will this time of visitation be, to the slandering Gentiles or to the Christians who do good?
In relation to the Gentiles, God's visitation may be understood both as fearsome and as merciful.
The time of fearsome visitation is the time of various calamities and sorrows, through which God's chastising judgment upon sinners is accomplished.
A time of calamities often compels even the most stubborn to turn to God and to judge more rightly concerning virtuous people who please God and concerning their good works.
In this sense one may understand the Apostle's words, that those who slander Christians as evildoers can be brought to their senses by calamities and glorify God when they see the Christians' good works.
Yet it accords more with the context to understand God's visitation as merciful toward the Gentiles. Then they understand the works of Christians as they should and glorify God precisely because of those works.
They perceive their whole beauty and greatness, and turn to faith and repentance (cf. Lk 19:44).
"By those who slander us the Apostle means the Gentiles, and by the day of visitation the investigation according to the laws of this world.
For when they examine our life and find that their notion of us contradicts reality, they themselves are corrected in their shameful deeds and thus glorify God" (Theophylact).
In relation to Christians themselves, God's visitation can be understood only as merciful. It means special benefactions of God toward believers.
Seeing these, the slanderers will understand them precisely as a visible reward for their good works and will glorify God for them, recognizing Him in such visitation as the true God.
Whichever of these meanings of the word "visitation" one accepts, the general thought of the saying remains clear.