"May there be with you," and so forth: in the greetings of the Apostle Peter to his readers, grace and peace are invoked upon them (1 Pet 1:2; 2 Pet 1:2). The same is true in most of the epistles of the Apostle Paul.
In some of Paul's epistles, however, as here, grace, mercy, and peace are invoked (1 Tim 1:2; 2 Tim 1:2; Tit 1:4).
Grace is the sum of all spiritual powers and gifts needed for progress in the pious life (cf. Eph 2:4-10; Rom 3:24).
Mercy is compassion, pity, or compassionate love toward the poor and humble man. It is necessary for him at all stages of his spiritual perfection, even the highest possible ones (Lk 10:30-37).
Peace is the repose of the spirit in God after it has been reconciled by the redemptive sacrifice of Christ the Savior (cf. 1 Pet 1:2).
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"From God," who is the Father both of the Lord Jesus and of all believers, and "from the Lord Jesus Christ," the God-man who redeemed the world and reconciled it with God.
"The Son of the Father": "for only God, in the proper sense, is Father of the Son" (Theophylact).
"In truth and love": this is a distinctive feature in the greeting of this apostle alone. It indicates in what, precisely, grace, mercy, and peace are revealed in believers, and for what they are given to them.
They are given for strengthening and perfecting in their spirit both truth and love. May truth and love be the ever-abiding principle of their life and activity in perfection.