There are many theological issues over which sincere Christians can agree to disagree until further light comes to us. Evangelical Christians have learned to be a united renewal movement within the broader Church despite disagreements over the form and mode of baptism, models of church government, Calvinism versus Arminian soteriology, speaking in tongues and so on. The two-decade long process of Evangelical and Catholics Together has dulled the anti-Catholic fervor of most Evangelicals, although many Reformed conservatives still hold out. Personally, I think a tipping point was reached with the Lutheran-Catholic joint statement on justification and the Catechism of the Catholic Church. I still believe the Roman Catholic Church is in need of further reforms, but I acknowledge that on many issues the last two centuries has witnessed a remarkable number of reforms for which the 16th century Reformers called. Significant ecumenical progress is underway.
The sheer number of such non-fellowship destroying theological issues and the good ecumenical news on several fronts, however, can have the unintended side-effect of inuring us to more serious theological disagreements which do, in fact, destroy the unity of Christ's Church and cause the loss of salvation for those who deny central truths of the Scripture. As Evangelicals and Catholics come closer together, liberal Protestantism becomes increasingly isolated from the mainstream of Christianity.
To deny the substitutionary atonement of Christ as necessary for the forgiveness of sins or the Triune nature of God or the full deity and humanity of the God-Man, Jesus Christ - these are not things which we can agree to disagree about. To deny these doctrines is to deny the Faith and to separate oneself from Christ and from his body in schism.
A serious question that must be faced today, given the slide into acceptance of so-called same-sex "marriage" by liberal Protestants, is whether or not this issue falls into the category of secondary doctrines about which disagreement can be tolerated without destroying the Church's unity or whether or not the endorsement of same-sex "marriage" is a soul-destroying heresy that rends the fabric of Christ's Church. Technically, a heresy is a doctrinal deviation that causes the heretic to be regarded as a non-Christian doomed to eternal punishment and which causes a schism in the Church by placing those who embrace the heretic outside the Church. Not all doctrinal disagreements rise to the level of heresies, but some do. It takes spiritual discernment, careful thought, historical awareness and, above all, deep biblical understanding to tell which are which.
So, is the embrace of the charade known today as same-sex "marriage" a heresy?
John Piper has a good post on Dietrich Bonhoffer and Wolfhart Pannenberg, two major twentieth-century theologians who have made solemn judgments on heresy and schism. Bonhoffer denounced the "Aryan Paragraph," which the Nazis tried to impose on the German Protestant Church in order to exclude Jews from the Church as heresy in the precise and fullest sense. In the Barmen Declaration (authored by Karl Barth) the Confessing Church separated itself from the German Christians who accepted the Aryan Paragraph. They did not just claim to be disagreeing with fellow-Christians; they were clear that to accept the Nazi demand that Jews be excluded from the Church was to abandon the Church and cease to have a credible Christian testimony.
Piper also quotes Wolfhart Pannenberg on the same-sex "marriage" issue as follows:
Here lies the boundary of a Christian church that knows itself to be bound by the authority of Scripture. Those who urge the church to change the norm of its teaching on this matter must know that they are promoting schism. If a church were to let itself be pushed to the point where it ceased to treat homosexual activity as a departure from the biblical norm, and recognized homosexual unions as a personal partnership of love equivalent to marriage, such a church would stand no longer on biblical ground but against the unequivocal witness of Scripture. A church that took this step would cease to be the one, holy, catholic, and apostolic church.
You can access Pannenberg's entire article, of which this is the final paragraph, here. Pannenberg views the embrace of same-sex "marriage" as a schismatic act. It is a heresy.
Piper notes that the Aryan Paragraph and the endorsement of homosexuality both deny the cross of Christ. Christ died, he notes, to bring Jews and Gentiles together in one body. (Eph. 2:14-16) And Christ died to bring repentant sinners into the Kingdom of God. (I Cor. 6:9-10) Therefore, he concludes, to exclude Jews is to deny Christ and his cross and to affirm a way of life that excludes people from the kingdom of God is to take a stand against the cross of Christ, which aims to save people for the Kingdom of God.
There is no plausible way to view those who endorse homosexual sin as attempting to acknowledge the authority of Scripture. Scripture is crystal clear that homosexual activity is sin and homosexual temptations are a result of a fallen, disordered, human sexuality - a curse we all bear. There is not a single positive or neutral reference to homosexuality in all of Scripture. Romans 1 distinguishes between those who are tempted and fall into temptation and those who "not only do them but give approval to those who practice them." (Rom. 1:32) Those who commit homosexual sins merely need to repent and they can be forgiven. But those who "give approval" to such sins need to change their minds about the morality of such actions before they can repent. It is far worse to teach that evil is good and that sinful acts are not under God's condemnation than merely to engage in a wrong act. The latter imperils one's own soul, but he former imperils the souls of others as well. Only a mind that is sunk deep into sin and rebellion against Christ could endorse same-sex "marriage" as good.
The United Church of Canada and the Anglican Church of Canada have gone into schism from the rest of Protestantism. It is highly problematic even to articulate a way in which they could legitimately be called Christian Churches anymore. Their status would be closer to the Mormans or Jehovah's Witnesses than to Protestant Churches. This is a very serious matter for those who find themselves members of schismatic bodies. They need our prayers as they wrestle with what to do.The tragedy of being conformed to this world is the loss of Jesus Christ. Same-sex "marriage" is probably the primary way contemporary Christians are being conformed to this world and thus drawn away from a living unity with our Lord and His Church. Clarity on this issue is a major requirement for effective pastoral ministry.