WHY do we say that Christians should rest in hope of Christ’s parousia?

For the Christian, the parousia will be a joyous royal visit. It is a visit of a different kind because the Blessed Visitor shall never thereafter depart. It is “The Day” for which the believer strains forward with eager anticipation; the day on which his tears shall be wiped away, and sorrow and sighing shall be no more.

Isaiah 35:4, 10: Say to them that are of a fearful heart, Be strong, fear not: behold, your God will come with vengeance, even God with a recompence; he will come and save you. ... And the ransomed of the LORD shall return, and come to Zion with songs and everlasting joy upon their heads: they shall obtain joy and gladness, and sorrow and sighing shall flee away.

For the wicked, it will be a day when God arrives in fire and declares His personal presence to a rebellious world which will then be under the deception and dominion of a man of lawlessness inspired by Satan. It is a day of judgement.

Isaiah 13:9-11: Behold, the day of the LORD cometh, cruel both with wrath and fierce anger, to lay the land desolate: and he shall destroy the sinners thereof out of it. For the stars of heaven and the constellations thereof shall not give their light: the sun shall be darkened in his going forth, and the moon shall not cause her light to shine. And I will punish the world for their evil, and the wicked for their iniquity; and I will cause the arrogancy of the proud to cease, and will lay low the haughtiness of the terrible.

Obadiah 1:15-16: For the day of the LORD is near upon all the heathen: as thou hast done, it shall be done unto thee: thy reward shall return upon thine own head. For as ye have drunk upon my holy mountain, so shall all the heathen drink continually, yea, they shall drink, and they shall swallow down, and they shall be as though they had not been.