The opening verses of Psalm 115 are:
Crédidi, propter quod locútus sum: * ego autem humiliátus sum nimis.
I have believed, therefore have I spoken; but I have been humbled exceedingly.
Ego dixi in excéssu meo: * Omnis homo mendax.
I said in my excess: Every man is a liar.
Crédidi (I have believed) propter quod (therefore/on that account) locútus sum (I have said/spoken) when I said)
credo, didi, ditum, ere 3, to believe, with the dative, or with in followed by the ace. or abl.; passive, to be
faithful to, loyally devoted to.
propter quod = propterea or quando, therefore, on that account.
loquor, locutus sum, loqui, to speak, utter, tell
ego (I) autem (but) humiliátus sum (I have been humbled) nimis (exceedingly/greatly)
humilio, avi, atum, are to humble, bring low.
nimis, adv., exceedingly, greatly, beyond measure.
Ego (I) dixi (I said) in (in) excéssu (dismay/excess/distress/consternation) meo (my)
excessus, us, m. a going out, going forth, departure; transport (terror, perturbation, emotion, dismay)
Omnis (all) homo (men) [are] mendax (liars)
mendax, acis , given or prone to lying, mendacious; deceitful, unreliable.
Faith calls forth action, in this case testimony, even though we may suffer for speaking up. Indeed, St John Chrysostom cites a version of the Greek that provides an alternate reading for the second phrase, 'Yet I was severely abused'. And St Augustine comments on the verse:
"For he suffered many tribulations, for the sake of the word which he faithfully held, faithfully preached; and he was sorely brought down; as they feared who loved the praise of men better than that of God."
The traditional resolution of the paradoxical nature of a man claiming that all men are liars is twofold. First, though all men lie, they do not lie all the time. St Jerome, for example, suggested:
"...It only takes one lie to be a liar, and even a regular liar may tell the truth on occasion. Had the statement been: 'Every man lies at every opportunity, and therefore not a word of truth is ever spoken by a man', a case could have been made for the existence of a paradox in what David reportedly said, although this also would probably have a perfectly rational explanation based entirely on semantics."
The other part of the resolution lies in the fact that Scripture is inspired directly by God: through grace we can see and speak truth. St Augustine comments:
"In consternation he has had regard to his infirmity, and has seen that he ought not to presume on himself; for as far as pertains to the man himself, he is a liar, but by the grace of God he is made true; lest yielding to the pressure of his enemies he might not speak what he had believed, but might deny it; even as it happened to Peter, since he had trusted in himself, and was to be taught that we ought not to trust in man. And if every one ought not to trust in man, surely not in himself; because he is a man. Rightly therefore in his fear did he perceive that every man was a liar; since they also whom no fear robs of their presence of mind, so that they never lie by yielding to the persecutors, are such by the gifts of God, not by their own strength...."
The reference to suffering is not just to the reaction of evil men to truth, but also to the transient suffering that God allows the would-be saint to suffer, the carrying of our cross, necessary to our spiritual development. Pope Benedict XVI for example comments:
The Apostle feels in spiritual harmony with the Psalmist, in serene trust and sincere witness, notwithstanding suffering and human weakness…. In our text the memory of a distressing past surfaces: the person praying has held high the torch of faith, even when on his lips played the bitterness of despair and unhappiness (cf. Ps 116: 10).
The point, as Pope Benedict XVI has pointed out, is that God saves us from the false culture around us that promotes a false concern with the present over eternal happiness:
Writing to the Romans, Paul takes up again verse two of the Psalm and highlights a difference between God who is faithful and man who is inconsistent: "God must be proved true even though every man be proved a liar" (Rom 3: 4)… Indeed, around him an icy curtain of hatred and deceit is being raised, as the neighbour shows himself to be false and unfaithful (cf. v. 11). The supplication, however, is now transformed into gratitude because the Lord has remained faithful in this context of infidelity and has saved his faithful [servant] from the dark vortex of lies (cf. v. 11).
Help us Lord when we struggle in the depths of unhappiness and distress, help us always to believe and speak your truth.
Help us Lord to carry our cross, to grow in humble realization of our own inadequacy; to trust in your perfecting grace.
Cassiodorus reminds us, in his commentary on the psalm, of St Benedict's 'Tool of Good Work' of attributing to God whatever good we see in ourselves, and to recognize that the evil is our own doing:
"Just as unreflective and foolish people lyingly say that they are most truthful, so enlightened persons know that they are liars. So this undeviating rule should bind us: when we lie through the frailty of the flesh, we should confess the fault as ours, but when we are truthful we should attribute it to God's gift."
1 Crédidi, propter quod locútus sum: * ego autem humiliátus sum nimis.
2 Ego dixi in excéssu meo: * Omnis homo mendax.
3 Quid retríbuam Dómino, * pro ómnibus, quæ retríbuit mihi?
4 Cálicem salutáris accípiam: * et nomen Dómini invocábo.
5 Vota mea Dómino reddam coram omni pópulo ejus: * pretiósa in conspéctu Dómini mors sanctórum ejus:
6 O Dómine, quia ego servus tuus: * ego servus tuus, et fílius ancíllæ tuæ.
7 Dirupísti víncula mea: * tibi sacrificábo hóstiam laudis, et nomen Dómini invocábo.
8 Vota mea Dómino reddam in conspéctu omnis pópuli ejus: * in átriis domus Dómini, in médio tui, Jerúsalem.
You can notes on the next verse here.