Was dead - This is capable of two significations:
1. "I supposed" that he was dead, but I know now that he is alive.
2. He was "dead to virtue" - he was sunk in pleasure and vice.
The word is not unfrequently thus used. See 1-Timothy 5:6; Matthew 8:22; Romans 6:13. Hence, to be restored to "virtue" is said to be restored again to life, Romans 6:13; Revelation 3:1; Ephesians 2:1. It is probable that this latter is the meaning here. See Luke 15:32.
Was lost - Had wandered away from home, and we knew not where he was.

Was dead - Lost to all good - given up to all evil. In this figurative sense the word is used by the best Greek writers. See many examples in Kypke.

For this my son was dead,.... These words contain the reasons of the above entertainment, and of all that spiritual joy and mirth; in which the father acknowledges the returning penitent as his son; though he had behaved so wickedly before, and though he judged himself unworthy of the relation; and this he did, by sending the Spirit of adoption into his heart, to witness his sonship to him; and takes notice of his past state and condition, to show the great reason there was for joy, at his present one: for before be was "dead", dead in Adam, in whom all died; dead in law, being under a sentence of condemnation and death; and dead in trespasses and sins, which is a spiritual or moral death: in which all mankind by nature are: and which lies in a separation from God, Father, Son, and Spirit; in an alienation from the life of God; in a deformation of the image of God; in a loss of original righteousness; in the darkness of the understanding; in the inordinateness of the affections; in the pollution of the mind and conscience; in the stubbornness of the will; and in an impotency to that which is spiritually good; and in a privation of spiritual sense and motion: this had been the case, but now it was otherwise:
and is alive again: the Spirit of life from Christ had entered into him, and Christ was formed in his heart; and a principle of life was infused into him; a divine image was enstamped upon his soul; the understanding was enlightened in divine things; the affections were set upon them; the will was subjected to God, to his will and law, and to Christ and his righteousness, and the way of salvation by him, and to his commands and ordinances; and principles of grace and holiness were wrought in him, to do as well as to will; a spiritual sense of things were given him; a spiritual sight, hearing, tasting, and feeling, and savouring; he lived a life of holiness from Christ, of faith upon him, and of communion with him, and to his glory: and he came to be so, not of himself, nor by any creature; for no man can quicken himself, nor can any creature do it for him; it was entirely owing to the power and grace of God: and great reason here was for joy and mirth, as there is for every one that is quickened by the Spirit of God; for such shall never enter into condemnation, nor die again, but shall live and reign with Christ for ever:
he was lost; lost in Adam, and in himself; so he was when in the far country, and when among the swine and husks; so as that he knew not where he was, nor what a condition he was in; nor did he know how to get out of it, nor could he help himself; nor could any other creature; though not irretrievably and irrecoverably lost; not to the love of God, his knowledge of him, care and provision for him in Christ, in his counsel and covenant: hence the following mercy,
and is found; not only by Christ, in redemption, but by the Spirit of Christ in the effectual calling; when he was brought and came to himself, and saw his lost state and condition by nature; and when he was directed and brought home to his Father's house, and entertained with all the provisions of it; and such have reason to rejoice and be glad, for they shall be found in Christ at death and at judgment, and shall be with him to all eternity:
and they began to be merry: all parties. The Father expressed his joy, and the gladness of his heart, upon the return of his son to him; he exhorted to be merry on this account, Luke 15:23 and enforces it with reasons in this verse, taken from the relation he stood in to him, and the wonderful change that had passed upon him, and the finding of him; and he rejoices himself at his conversion, in the exercise of that grace which he himself implanted, and in the performance of duty by his assistance: not that any new joy arises in God's heart at such a time; for he always rejoiced in the persons of his elect, as they were the objects of his love, as chosen in Christ, and given to him, and as interested in the covenant of grace; and he rejoiced in the accomplishment of their salvation, by his Son: but in conversion, there are new expressions of joy; he rejoices over them to do them good, and rejoices in the good he does them; and this is the open beginning of his joy, and but the beginning of it; for it will continue, it is not all over, not all expressed, but will be in the fullest manner hereafter, to all eternity: the returned son began to be merry, as he had good reason for it; as that he was come back from the far country, where a mighty famine had been: and from the citizen of that country, his fields, and swine; that he was come to his father, and his father's house, where was bread enough and to spare; an house well furnished with all suitable provisions; a family made up of saints, where ministers of the Gospel are stewards, and angels guards, and where Christ is Son, priest, and master; and that he was received here, and owned as a son; not only was one secretly, but was owned as such openly; and was not only called so by the servants, but by the father himself; and that after he had behaved so vilely, and in his own conscience knew he was unworthy of the relation; and that he was received immediately, as soon as ever he came, and that in the most tender manner; and was entertained in the most free, generous, and sumptuous way; though he went away from his father of himself, and had spent his substance in a scandalous manner; and was in a most filthy, ragged, and piteous condition; and that he should be clothed with the best robe, the robe of Christ's righteousness; and so had nothing to fear from law and justice; nor was he in any danger of wrath to come because of his sins; nor had he any reason to doubt of his right and admission to the heavenly glory; and that he had the ring of love on the hand of faith, and could believe his interest in it, which is better than life, and will continue for ever; and that his feet were shod with the preparation of the Gospel of peace; that he understood the Gospel, and was brought to a submission to Gospel ordinances, and had his conversation agreeably to it; and that the fatted calf was killed, and set before him to eat of, and feed upon: and now he began to live and fare sumptuously, and to have spiritual joy and pleasure, which he never knew before; and this was but the beginning of joy to him: spiritual joy is not all over at once, it continues and increases; nor is it full and perfect in this life, but in heaven it will be complete, and without interruption; the servants also, the ministers of the Gospel, began to be merry on this occasion; who express their joy at the conversion of sinners, because of the glory of Father, Son, and Spirit, concerned in it; because of the grace bestowed then on sinners themselves; and because the interest of Christ is strengthened, and his churches increased, and Satan's kingdom weakened; and because their own ministry is blessed; and which strengthens their hands and hearts, and encourages them to go on in their work: and this is but the beginning of their joy; for they continue to rejoice at the growth of grace in believers, and when they are in a thriving and flourishing condition; when they walk becoming the Gospel of Christ, and live in peace among themselves; and persevere in faith and holiness to the end; and these will be their joy and crown of rejoicing, at the coming of Christ Jesus.

my son--now twice his son.
dead . . . lost--to me; to himself--to my service, my satisfaction; to his own dignity, peace, profit.
alive again . . . found--to all these.
merry--(See on Luke 15:10).