Does the Catholic Church oppose universalism? I've been reading George MacDonald as of late and have heard in a few places that he argues for universalism. I though that had something to do with unitarianism, but in looking it up a tad more, I guess it means the idea that everyone will be saved.

Is this theology considered a heresy to the Catholic Church? If so, when where and why was it condemned?
According to this article (and I'm not expert enough to verify the reliability) the answer seems to be "not any more."


  A Summary and Some Resources
  
  The doctrine of universal salvation (also known as Apokatastasis or
  Apocatastasis) has usually been considered through the centuries to be
  heterodox but has become orthodox. It was maintained by the Second
  Vatican Council and by Pope John Paul II and it is promoted in the new
  Catechism of the Catholic Church and in the post-Vatican II liturgy.


It was supported by Pope John Paul II  the following are three quotes from him.


  
  Eternal damnation remains a possibility, but we are not granted, without special divine revelation, the knowledge of whether or which
  human beings are effectively involved in it. (General Audience of July
  28, 1999)
  Christ, Redeemer of man, now for ever ‘clad in a robe dipped in blood’ (Apoc, 19,13), the everlasting, invincible guarantee of
  universal salvation. (Message of John Paul II to the Abbess General of
  the Order of the Most Holy Saviour of St Bridget)
  If the Holy Spirit, the Paraclete, is to convince the world precisely of this ‘judgment,’ undoubtedly he does so to continue
  Christ’s work aimed at universal salvation. We can therefore conclude
  that in bearing witness to Christ, the Paraclete is an assiduous
  (though invisible) advocate and defender of the work of salvation, and
  of all those engaged in this work. He is also the guarantor of the
  definitive triumph over sin and over the world subjected to sin, in
  order to free it from sin and introduce it into the way of salvation.
  (General Audience of May 24, 1989)
  


As for the statement that the author claims that the teaching is found in the catachism, here's the rationale:


  The new, post-Vatican II Catechism of the Catholic Church also gives
  us to hope that all will be saved.
  
  
  1058 The Church prays that no one should be lost: ‘Lord, let me never be parted from you.’ If it is true that no one can save himself,
  it is also true that God ‘desires all men to be saved’ (1 Tim 2:4),
  and that for him ‘all things are possible’ (Mt 19:26).
  1821 We can therefore hope in the glory of heaven promised by God to those who love him and do his will. In every circumstance, each one
  of us should hope, with the grace of God, to persevere ‘to the end’
  and to obtain the joy of heaven, as God’s eternal reward for the good
  works accomplished with the grace of Christ. In hope, the Church prays
  for ‘all men to be saved.’
  


Although in reading those myself, I don't know that they teach that universalism is true but rather that they have hope that it is.  But, clearly, if the have hope that it's true, it's not opposition to the teaching.  And much searching has failed to turn up recent statement denouncing universalism.