What does it mean for believers to stand together for the gospel?
In simple terms, it means that while they might have other theological differences, they are united in affirming the gospel’s core tenants. Specifically, they agree that sinners are justified not by their own efforts, but by grace alone, through faith alone, in Christ alone.
It’s that last element in particular that offends and annoys our pluralistic, inclusive society. But as F. F. Bruce explains, the gospel’s exclusivity flows directly out of Christ’s nature in His incarnation.
He is, in fact, the only way by which men and women may come to the Father, there is no other way. If this seems offensively exclusive, let it be borne in mind that the one who makes this claim is the incarnate Word, the revealer of the Father. If God has no avenue of communication with mankind apart from his Word . . . mankind has no avenue of approach to God apart from that same Word, who became flesh and dwelt among us in order to supply such an avenue of approach. 
Standing together for the gospel, then, is standing in agreement with Christ’s own assertions to His uniqueness: “I am the way, and the truth, and the life; no one comes to the Father but through Me” (John 14:6). It’s echoing the words Peter boldly proclaimed to the Sanhedrin, that “there is salvation in no one else; for there is no other name under heaven that has been given among men by which we must be saved” (Acts 4:12).
In spite of the innate exclusivity of the gospel, the world’s religions attempt to position themselves as co-laborers with Christianity. They might highlight similar stances on social issues, or simply try to identify a common enemy—whatever it takes to present the illusion of unity.
Worse still, many Christians are all too happy to lend those false religions spiritual credibility by operating as cobelligerents.
Such ecumenical partnerships require a muddying of doctrinal waters. Theological distinctives are downplayed or set aside in the name of unity, as both sides come to a polemical cease-fire in pursuit of a common goal.
The 1994 ecumenical treatise Evangelicals and Catholics Together: The Christian Mission in the Third Millennium (ECT) is a prime example. In an effort to kick-start “a springtime of world missions,” influential leaders from both faiths attempted to identify and affirm theological common ground for the sake of furthering the reach of the gospel (you can read the full document here).
In reality, they ignored centuries of church history and asserted vague platitudes about unity in Christ.
All who accept Christ as Lord and Savior are brothers and sisters in Christ. Evangelicals and Catholics are brothers and sisters in Christ. We have not chosen one another, just as we have not chosen Christ. He has chosen us, and he has chosen us to be his together (John 15). However imperfect our communion with one another, however deep our disagreements with one another, we recognize that there is but one church of Christ. There is one church because there is one Christ and the church is his body. However difficult the way, we recognize that we are called by God to a fuller realization of our unity in the body of Christ.
But what gospel were they uniting behind? Let’s not forget or ignore—as the signatories of ECT must have—that Catholic dogma pronounces anathema on anyone who preaches justification by faith alone. Here is the stark condemnation, spelled out by the Council of Trent:
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Source: Together Against Ecumenism