In Catholic doctrine, there is a Baptism of desire, where God grants the Grace of Baptism to those who desire it, but are prevented by death from obtaining it. In the same way, those who seek Truth, will be given the Grace to find it. "Ask, and it shall be given you; seek, and ye shall find; knock, and it shall be opened unto you." [Matt 7:7] Even if they do not discover the whole truth in this life, God honors the desire. I call this the "Gospel of Desire".
How is this different from the heresy? The difference is that in the Catholic version, the seeker get progressively closer to the Truth, eventually becoming a Catholic. In the Liberal version, all religions are equally valid; there is no reason to move toward any absolute Truth.
Apart from Liberals and Calvinists, Protestant ecumenism is similar to the Catholic. But in the Protestant picture, the absolute Truth we are all seeking is not represented by any one denomination.
The Catholic, Protestant, and Liberal versions can be summarized as Seeking Catholic Truth, Seeking Common Truth, and Avoiding Common Truth.
In Calvinism, God gives Grace to seek the Truth to those He chooses - and they cannot resist His Grace. For Arminians (Catholic and Protestant), God gives Grace to seek the Truth to everyone, but they can resist Grace.