1 Corinthians 15:29
"Else what shall they do which are baptized for the dead, if the dead rise not at all? why are they then baptized for the dead?"
So, how about the justification for purgatory in the Bible? I've not heard it before.
There is plenty, but I will use this:
And whosoever shall speak a word against the Son of man, it shall be forgiven him: but he that shall speak against the Holy Ghost, it shall not be forgiven him, neither in this world, nor in the world to come. (Matthew 12:32)
It is simply a matter of what is "the world to come." There is no longer any need for forgiveness in Heaven for all has already been forgiven, there is no possibility of forgiveness in Hell, so that means there must be a THIRD "world to come" where forgiveness is both still needed AND still available -- this is Purgatory.
Who's "they"? It doesn't say "we" or "you" or "the church"; it says "they". We don't know who "they" were.
The Greek also does not demand proxy baptism for the dead; it's more ambiguous than that. There's zero support among the church fathers for the practice AFAIK.
As for purgatory, 1 Cor 3:15 and the immediate vicinity. "Saved, but as through fire" sounds like purgatory to me.
For you and are other non-Catholic friends, Purgatory has to be understood in the context of how Catholics understand Grace and sin. Sin ruptures and breaks our communion with God and it is Grace that justifies us and makes us Holy. Thus, Grace, from the Catholic perspective is transformative and not just a covering of Gods Grace, which is the classic Protestant understanding. The Catechism discusses Grace in paragraph 1996 and 1997:
CCC 1996 Our justification comes from the grace of God. Grace is favor, the free and undeserved help that God gives us to respond to his call to become children of God, adoptive sons, partakers of the divine nature and of eternal life.
CCC 1997 Grace is a participation in the life of God. It introduces us into the intimacy of Trinitarian life: by Baptism the Christian participates in the grace of Christ, the Head of his Body. As an “adopted son” he can henceforth call God “Father,” in union with the only Son. He receives the life of the Spirit who breathes charity into him and who forms the Church.
The Catechism states that as sanctifying Grace, God shares his divine life and friendship with us in a habitual gift, a stable and supernatural disposition that enables the soul to live with God and act by his love. As actual grace, God gives us the help to conform our lives to his will.
With respect to Purgatory, the Catechism states:
CCC 1031 The Church gives the name Purgatory to this final purification of the elect, which is entirely different from the punishment of the damned. The Church formulated her doctrine of faith on Purgatory especially at the Councils of Florence and Trent. The tradition of the Church, by reference to certain texts of Scripture, speaks of a cleansing fire:
CCC 1031-contnued: As for certain lesser faults, we must believe that, before the Final Judgment, there is a purifying fire. He who is truth says that whoever utters blasphemy against the Holy Spirit will be pardoned neither in this age nor in the age to come. From this sentence we understand that certain offenses can be forgiven in this age, but certain others in the age to come.
So, when we see Christ state that only the pure of heart shall see God (cf. Mt 5:8) amd that nothing unclean shall enter heaven (cf Rev 21: 27). The CCC in para 1031 refers to Mt 12:32, which states that some sins will not be forgiven in this age or the age to come and St. Paul in 1 Cor 3:15 states that but if someones work is burned up, that one will suffer loss; the person will be saved, but only as through fire. A point here is that the Latin word purgation means to pass through fire, which is where the word purgatory comes from.
We also see in 1 Pet 3: 19 we see that Christ went to speak to the spirits in prison and later in 1 Pet 4:6 we read For this is why the gospel was preached even to the dead that condemned in the flesh in human estimation, they might live in the spirit in the estimation of God. While these passages dont prove purgatory, they do imply a spiritual state that is not hell or Heaven. Finally, we also read 2 Mac 12:46 that prayers were offered for the dead, which of course is in the Catholic and Orthodox OT canons, but not Protestant.
In summary, the doctrine of purgatory is consistent with the Catholic understanding of Grace and Sin and is supported by Sacred Scripture. In addition, the Sacred Tradition of the Church, as confirmed by the Church Fathers also supports the doctrine of Purgatory as prayers for the dead all clearly taught by the Fathers. For example,
ST Cyril of Jerusalem writes:
Then we make mention also of those who have already fallen asleep: first, the patriarchs, prophets, apostles, and martyrs, that through their prayers and supplications God would receive our petition; next, we make mention also of the holy fathers and bishops who have already fallen asleep, and, to put it simply, of all among us who have already fallen asleep. For we believe that it will be of very great benefit to the souls of those for whom the petition is carried up, while this holyand most solemn sacrifice is laid out” (Catechetical Lectures 23:5:9 [A.D. 350]).
ST. GREGORY OF NYSSA writes:
“If a man distinguish in himself what is peculiarly human from that which is irrational, and if he be on the watch for a life of greater urbanity for himself, in this present life he will purify himself of any evil contracted, overcoming the irrational by reason. If he have inclined to the irrational pressure of the passions, using for the passions the cooperating hide of things irrational, he may afterward in a quite different manner be very much interested in what is better, when, after his departure out of the body, he gains knowledge of the difference between virtue and vice and finds that he is not able to partake of divinity until he has been purged of the filthy contagion in his soul by the purifying fire” (Sermon on the Dead [A.D. 382]).
ST. JOHN CHRYSOSTOM writes:
“Let us help and commemorate them. If Jobs sons were purified by their fathers sacrifice [Job 1:5], why would we doubt that our offerings for the dead bring them some consolation? Let us not hesitate to help those who have died and to offer our prayers for them” (Homilies on 1 Corinthians 41:5 [A.D. 392]).
ST. Augustine writes:
There is an ecclesiastical discipline, as the faithful know, when the names of the martyrs are read aloud in that place at the altar of God, where prayer is not offered for them. Prayer, however, is offered for other dead who are remembered. It is wrong to pray for a martyr, to whose prayers we ought ourselves be commended (Sermons 159:1 [A.D. 411]).
But by the prayers of the holy Church, and by the salvific sacrifice, and by the alms which are given for their spirits, there is no doubt that the dead are aided, that the Lord might deal more mercifully with them than their sins would deserve. The whole Church observes this practice which was handed down by the Fathers: that it prays for those who have died in the communion of the body and blood of Christ, when they are commemorated in their own place in the sacrifice itself; and the sacrifice is offered also in memory of them, on their behalf. If, then, works of mercy are celebrated for the sake of those who are being remembered, who would hesitate to recommend them, on whose behalf prayers to God are not offered in vain? It is not at all to be doubted that such prayers are of profit to the dead (ibid., 172:2).
Temporal punishments are suffered by some in this life only, by some after death, by some both here and hereafter, but all of them before that last and strictest judgment. But not all who suffer temporal punishments after death will come to eternal punishments, which are to follow after that judgment (The City of God 21:13 [A.D. 419]).
That there should be some fire even after this life is not incredible, and it can be inquired into and either be discovered or left hidden whether some of the faithful may be saved, some more slowly and some more quickly in the greater or lesser degree in which they loved the good things that perish, through a certain purgatorial fire (Handbook on Faith, Hope, and Charity 18:69 [A.D. 421]).
While you may not agree with the doctrine of Purgatory, the Catholic Churchs doctrine is well grounded in both Sacred Scripture and Sacred Tradition.
Hope this helps