3 But the saints, and they who truly practise virtue, 'mortify their members which are upon the earth, fornication, uncleanness passions, evil concupiscence;' and, as the result of this, are pure and without spot, confiding in the promise of our Saviour, who said, 'Blessed are the pure in heart, for they shall see God.' These, having become dead to the world, and renounced the merchandise of the world, gain an honourable death; for, 'precious in the sight of the Lord is the death of His saints. ' They are also able, preserving the Apostolic likeness, to say, 'I am crucified with Christ, nevertheless I live; yet not I, but Christ lives in me.'
For that is the true life, which a man lives in Christ; for although they are dead to the world, yet they dwell as it were in heaven, minding those things which are above, as he who was a lover of such a habitation said, 'While we walk on earth, our dwelling is in heaven. ' Now those who thus live, and are partakers in such virtue, are alone able to give glory to God, and this it is which essentially constitutes a feast and a holiday. For the feast does not consist in pleasant intercourse at meals, nor splendour of clothing, nor days of leisure, but in the acknowledgment of God, and the offering of thanksgiving and of praise to Him. Now this belongs to the saints alone, who live in Christ; for it is written, 'The dead shall not praise You, O Lord, neither all those who go down into silence; but we who live will bless the Lord, from henceforth even for ever. ' So was it with Hezekiah, who was delivered from death, and therefore praised God, saying, 'Those who are in hades cannot praise You; the dead cannot bless You; but the living shall bless You, as I also do.'
For to praise and bless God belongs to those only who live in Christ, and by means of this they go up to the feast; for the Passover is not of the Gentiles, nor of those who are yet Jews in the flesh; but of those who acknowledge the truth in Christ, as he declares who was sent to proclaim such a feast; 'Our Passover, Christ, is sacrificed.'
Source: Letters (New Advent)