The fallacy of most ecumenical proponents is that they misinterpret or fail to understand the Biblical truth.
Christians are no better than those of other religions. In fact, the Apostle Paul called himself the chiefest of sinners. Christians are those who have believed the God of creation, while those of other religions are practicing man-made institutions or demonic instruction introduced over the years.
23 For the wages of sin is death; but the gift of God is eternal life through Jesus Christ our Lord. Romans 6:23
He that believeth on him is not condemned: but he that believeth not is condemned already, because he hath not believed in the name of the only begotten Son of God.
John 3:18
Jesus gave the command to go into all the world teaching all nations to observe all things that Jesus commanded, and to baptize them in the name of the Father, Son, and of the Holy Ghost.
19 Go ye therefore, and teach all nations, baptizing them in the name of the Father, and of the Son, and of the Holy Ghost: 20 Teaching them to observe all things whatsoever I have commanded you: and, lo, I am with you alway, even unto the end of the world. Amen. Matthew 28:19-20
No matter whether individuals have heard of Jesus or not, they have no excuse because the things of God are revealed to each one by the creation.
16 For I am not ashamed of the gospel of Christ: for it is the power of God unto salvation to every one that believeth; to the Jew first, and also to the Greek. 17 For therein is the righteousness of God revealed from faith to faith: as it is written, The just shall live by faith. 18 For the wrath of God is revealed from heaven against all ungodliness and unrighteousness of men, who hold the truth in unrighteousness;
19 Because that which may be known of God is manifest in them; for God hath shewed it unto them. 20 For the invisible things of him from the creation of the world are clearly seen, being understood by the things that are made, even his eternal power and Godhead; so that they are without excuse: 21 Because that, when they knew God, they glorified him not as God, neither were thankful; but became vain in their imaginations, and their foolish heart was darkened. 22 Professing themselves to be wise, they became fools, 23 And changed the glory of the uncorruptible God into an image made like to corruptible man, and to birds, and fourfooted beasts, and creeping things. 24 Wherefore God also gave them up to uncleanness through the lusts of their own hearts, to dishonour their own bodies between themselves: 25 Who changed the truth of God into a lie, and worshipped and served the creature more than the Creator, who is blessed for ever. Amen. 26 For this cause God gave them up unto vile affections: for even their women did change the natural use into that which is against nature: 27 And likewise also the men, leaving the natural use of the woman, burned in their lust one toward another; men with men working that which is unseemly, and receiving in themselves that recompence of their error which was meet. 28 And even as they did not like to retain God in their knowledge, God gave them over to a reprobate mind, to do those things which are not convenient; 29 Being filled with all unrighteousness, fornication, wickedness, covetousness, maliciousness; full of envy, murder, debate, deceit, malignity; whisperers, 30 Backbiters, haters of God, despiteful, proud, boasters, inventors of evil things, disobedient to parents, 31 Without understanding, covenantbreakers, without natural affection, implacable, unmerciful: 32 Who knowing the judgment of God, that they which commit such things are worthy of death, not only do the same, but have pleasure in them that do them.
Romans 1:16-32