The fifth Beatitude in Matthew (Matt 5:7) is probably the simplest and most straightforward of the collection:
Maka/rioi oi( e)leh/mone$, o%ti au)toi\ e)lehqh/sontai
“Happy the merciful/compassionate (ones), (in) that they will receive mercy/compassion”
The adjective e)leh/mwn (ele¢¡mœn, “merciful, compassionate”) is rare in the New Testament, occurring elsewhere only in Heb 2:17 (in the context of Christ’s humanity and his role as High Priest of God). The noun e&leo$ (éleos, “mercy, compassion”) is more common, usually referring to the mercy of God shown to His chosen people (Luke 1:50, 54, 58, 72, 78; Rom 9:23; 11:31; 15:8; Eph 2:4; 2 Tim 1:16, 18; 1 Pet 1:3, etc). Jesus uses the noun only in Matt 9:13; 12:7 (of God, quoting Hos 6:6, “I wish [for] mercy, not sacrifice”) and again in Matt 23:23. The verb e)lee/w (eleéœ, “[show/have] mercy, compassion”) is also used regularly in reference to God’s mercy (Mark 5:19; Rom 9:15 [quoting Exod 33:19]; 11:30-32; 2 Cor 4:1; Phil 2:27), the passive form itself (divine passive) often implying divine mercy (1 Tim 1:13, 16; 1 Pet 2:10). In the Gospels, the 2nd person imperative e)lehson (“have mercy [on me/us]…!”) appears where persons in need call out to God (or Jesus, Mark 10:47-48 par, etc). The derived noun e)lehmosu/nh (ele¢mosún¢, “mercifulness”) came to be used in the sense of “act/expression of mercy”, with the technical sense of giving alms, charity for the poor and needy, etc—as such the word held an important place in Jewish religion (see Matt 6:2-4; Acts 10:2, 4, 31). The verb oi)ktirme/w (with related noun/adjective oi)ktirmo/$, oi)kti/rmwn) is more or less synonymous with e)lee/w. (cp. Matt 5:7 and Luke 6:36, see below).
The predominant motif is that of the mercy and compassion of God in dealing with His people, as expressed often throughout the Old Testament (of many references, see Exod 33:19; 34:6; Deut 4:31; 2 Sam 22:26; Neh 9:17, 19, 27-28, 31; Psalm 69:16; 86:15; 111:4; 116:5; 145:8-9; Lam 3:22). The idea of God’s mercy upon sinners is not specified as such, but is implied in references in the Prophets to God restoring the fortunes of Israel and returning his favor following the cycle of disobedience-judgment-repentance (cf. Isa 19:22; 30:18; 60:10; Jer 3:12; 31:9, 20; 33:26; Hos 1:7; 2:1ff; Joel 2:13; Hab 3:2; Dan 9:9; Zech 1:16; 12:10; extended to the pagan world in Dan 4:27).
In this light, the religious and ethical obligation to act with mercy and compassion was reciprocal, summed up in 2 Sam 22:26 / Psalm 18:25:
“With the kind/merciful [dys!j*] (man) You show yourself kind/merciful, with the pure/complete [<ym!T*] man You show yourself pure/complete”
This is very much the idea present in the Beatitude—those who are merciful (here in this life) will receive mercy from God (in the life to come, i.e. in the Judgment). The relation between mercy/compassion and purity/competeness expressed in the Psalm underlies Jesus’ teaching as well. Note the following parallels (here the Matthean and Lukan forms complement one another):
“Happy the merciful [e)leh/mwn] ones, (in) that they will receive mercy”
“Become merciful [oi)kti/rmwn], even as [also] your Father is merciful”
“Therefore you shall be complete, as your heavenly Father is complete”
This reciprocal formula is closely connected to Jesus’ teaching on forgiveness (Matt 6:14-15; 18:21-22; Mk 11:25; Lk 6:37; 11:4), as illustrated in the parable of the Unforgiving Servant (Matt 18:23-35). Indeed, a number of Jesus’ parables relate to the theme of mercy and compassion (whether of God or the faithful disciple)—cf. Luke 10:29-37; 14:12-24; 15:3-7, 11-32; 16:19-22ff. The closest New Testament parallel to this teaching by Jesus is found in the letter of James (Jas 2:13, cf. also in 3:13-18), with similar exhortations to forgiveness in 2 Cor 2:10; Eph 4:32; Col 3:13; Jude 21-23. The impact of this teaching, especially as expressed here in the Beatitude, would have considerable influence on ethical instruction in the early Church—cf. 1 Clement 13:2 / Polycarp 2:3 (“be merciful, that you may receive mercy”), and Didache 3:8, etc.