6 Therefore the righteous and faithful servants of the Lord, who 'are made disciples for the kingdom of heaven, and bring forth from it things new and old;' and who 'meditate on the words of the Lord, when sitting in the house, when lying down or rising up, and when walking by the way;'— since they are of good hope because of the promise of the Spirit which said, 'Blessed is the man that has not walked in the counsel of the ungodly, nor stood in the way of sinners, nor sat in the seat of corrupters; but his delight is in the law of the Lord, and in His law does he meditate day and night;'— being grounded in faith, rejoicing in hope, fervent in spirit, they have boldness to say, 'My mouth shall speak wisdom, and the meditation of my heart shall be of understanding.'
And again, 'I have meditated on all Your works, and on the work of Your hands has been my meditation.' And, 'If I have remembered You on my bed, and in the morning have meditated on You. ' Afterwards, advancing in boldness, they say, 'The meditation of my heart is before You at all times. ' And what is the end of such an one? He cites immediately; 'The Lord is my Helper and my Redeemer. ' For to those who thus examine themselves, and conform their hearts to the Lord, nothing adverse shall happen; for indeed, their heart is strengthened by confidence in the Lord, as it is written, 'They who trust in the Lord are as mount Sion: he who dwells in Jerusalem shall not be moved for ever. ' For if at any time, the crafty one shall be presumptuously bold against them, chiefly that he may break the rank of the saints, and cause a division among brethren; even in this the Lord is with them, not only as an avenger on their behalf, but also when they have already been beaten, as a deliverer for them.
For this is the divine promise; 'The Lord shall fight for you.' Henceforth, although afflictions and trials from without overtake them, yet, being fashioned after the apostolic words, and 'being steadfast in tribulations, and persevering in prayers ' and in meditation on the law, they stand against those things which befall them, are well-pleasing to God, and give utterance to the words which are written, 'Afflictions and distresses have come upon me; but Your commandments are my meditation. '
Source: Letters (New Advent)