Inclusivism, one of several approaches to understanding the relationship between religions, asserts that while one set of beliefs is absolutely true, other sets of beliefs are at least partially true. It stands in contrast to exclusivism, which asserts that only one way is true and all others are in error. It is a particular form of religious pluralism, though that term may also assert that all beliefs are equally valid within a believer's particular context.
Broadly speaking, there are two schools of Inclusivist thought:
Traditional Inclusivism, which asserts that the believer's own views are absolutely true, and believers of other religions are correct insofar as they agree with that believer.
Relativistic Inclusivism, which asserts that an unknown set of assertions are Absolutely True, that no human being currently living has yet ascertained Absolute Truth, but that all human beings have partially ascertained Absolute Truth.
Strands of both types of Inclusivist thought run through all faiths.
Interpretatio graeca the common tendency of ancient Greek writers to equate foreign divinities to members of their own pantheon. Herodotus, for example, refers to the ancient Egyptian gods Amon, Osiris and Ptah as "Zeus," "Dionysus" and "Hephaestus." This could be seen an example of inclusivism, as could syncretism.
Syncretism functionized as an essential feature of Ancient Greek religion. Overall, Hellenistic culture in the age that followed Alexander the Great itself showed syncretist features, essentiially blending of Persian, Anatolian, Egyptian (and eventually Etruscan-Roman) elements within an Hellenic formula. The Egyptian god Amun developed as the Hellenized Zeus Ammon after Alexander the Great went into the desert to seek out Amun's oracle at Siwa.
Jesus said, [Paraphrase]"He who is not against me is for me." [Original]"for whoever is not against us is for us." (NIV) Gospel of Mark 9:40.
The Apostle Peter wrote of God: "He is patient with you, not wanting anyone to perish, but everyone to come to repentance." 2 Peter 3:9 (NIV)
"That was the true Light which gives light to every man coming into the world." John 1:9 Similarly Titus 2:11 says, "The grace of God that brings salvation has appeared to all men."
God loved the entire world and Jesus came to save it, not condemn it (John 3:16, 17.)
An aphorism common in some Christian circles: "All Truth is God's Truth."
Some Evangelical scholars believe that God judges all people based on their response to the Holy Spirit, and that just as Romans 2:14-15 shows that God is righteous by condemning people who violate natural law as they understand it, it also shows His mercy in forgiving those who have lived up to all the light they have had. Thus, it is possible for people to be saved through hearing the Gospel message of forgiveness of sins by Christ, even if they have not been instructed by Christian missionaries.
Psalm 19 presents general revelation, as exemplified by the sky and sun, in parallel with conversion. Verses 1-6 show the transcending of the barriers of language and geography. Verses 7-8 declare that the internalizing of the perfect law of the LORD can be efficacious in "converting the soul…making wise the simple…rejoicing the heart…enlightening the eyes."
Melchizedek (Genesis 14:18) and the Wise Men (Matthew 2:1-13) are examples of people who believed in God even though they were not part of the covenant people.
Cornelius already believed in God before Peter came and preached to him (Acts 10:1-48.) "Then Peter opened his mouth and said: 'In truth I perceive that God shows no partiality, But in every nation whoever fears Him and works righteousness is accepted by Him." Acts 10:34-35
The parable of the sheep and the goats (Matthew 25:31-46) portrays the judgment of the nations as being based on each individual's compassion on others, not on their religious background. The blessings pronounced upon the poor in spirit, the meek, the merciful, the peacemakers, those hungering for righteousness, etc. (Matthew 5:3-10) can also be understood as applying without reference to religion. Similarly, James 1:27 says, "Pure and undefiled religion before God and the Father is this: to visit orphans and widows in their trouble, and to keep oneself unspotted from the world."
Paul said that the Greeks had been worshiping God without knowing it. He said that in their semi-enlightened condition, they might grope for God and find Him, since He was not far from each one of us. Their own poets had declared that they were God's offspring. This shows that He was somewhat known to them. Acts 17:23-28
Jesus, speaking to a Samaritan woman in (John 4:22), said, "You worship what you do not know; we worship what we know, for salvation is from the Jews," implying, inclusivists believe, that it is possible to worship the true God without explicitly knowing it. Later, when an expositor of the Jewish laws asked him, “What shall I do to inherit eternal life?,” Jesus told the story of the good Samaritan and said, “Go and do likewise.” Luke 10:25-37
And he said, "Therefore, as through one man's offense judgment came to all men, resulting in condemnation, even so through one Man's righteous act the free gift came to all men, resulting in justification of life." Romans 5:18
Supporters of inclusivism include C. S. Lewis,John Wesley,Clark Pinnock,Karl Rahner, John E. Sanders, Terrance L. Tiessen (Reformed) and Robert Brush (contributor to The Arminian Magazine). While Billy Graham faithfully preached "salvation by faith in Christ alone" throughout his 60 year ministry as an evangelist, he has recently made controversial comments that border on inclusivism (but he does not like to refer to it by the term, because he is concerned that many people mean universalism when they refer to inclusivism). Graham has said, “I used to play God but I can’t do that any more. I used to believe that pagans in far-off countries were lost and were going to hell—if they did not have the Gospel of Jesus Christ preached to them. I no longer believe that,” he said carefully. “I believe that there are other ways of recognizing the existence of God—through nature, for instance—and plenty of other opportunities, therefore, of saying ’yes’ to God.”
The doctrine of inclusivism is held by Roman Catholics  and Seventh-day Adventists, asserting that while Christianity is the one true faith, other faiths are at least partially true, and therefore are valid ways of reaching salvation until the Gospels can be preached to them..
The Talmud states: "The righteous of all peoples have a place in the World-To-Come" (Tos. to Sanhedrin 13:2, Sifra to Leviticus 19:18), and affirms that the great majority of non-Jewish humanity will be saved, due to God's overwhelming mercy (BT Sanhedrin 105a).
The Torah mentions a number of righteous gentiles, including Melchizedek who presided at offerings to God that Abraham made (Gen. 14:18), Job of the land of Uz who had a whole book of the Hebrew Bible devoted to him as a paragon of righteousness beloved of God (see the book of Job), and the Ninevites, the people given to cruelty and idolatry could be accepted by God when they repented (see the Book of Jonah).
Rabbinic tradition asserts that the basic standard of righteousness was established in a covenant with Noah: anyone who keeps the seven commandments of this covenant is assured of salvation, no matter what their religion. This is standard Jewish teaching for the past two thousand years.
The Qur'an, revealed through Muhammad, states, "Those with Faith, those who are Jews, and the Christians and Sabaeans, all who have Faith in Allah and the Last Day and act rightly, will have their reward with their Lord. They will feel no fear and will know no sorrow." (Qur'an, Surat al-Baqara; 2:62)
Say, 'People of the Book! come to a proposition which is the same for us and you - that we should worship none but Allah and not associate any partners with Him and not take one another as lords besides Allah.' If they turn away, say, 'Bear witness that we are Muslims.'(Surah Al 'Imran; 3:64)
Today all good things have been made halal for you. And the food of those given the Book is also halal for you and your food is halal for them. So are chaste women from among the muminun and chaste women of those given the Book before you, once you have given them their dowries in marriage, not in fornication or taking them as lovers. But as for anyone who rejects iman, his actions will come to nothing and in the akhira he will be among the losers. (Surat al-Ma'ida: 5:5)
Among the people of the Book there are some who have iman in Allah and in what has been sent down to you and what was sent down to them, and who are humble before Allah. They do not sell Allah's Signs for a paltry price. Such people will have their reward with their Lord. And Allah is swift at reckoning. (Surah Al 'Imran; 3:199)
Call to the way of your Lord with wisdom and fair admonition, and argue with them in the kindest way. Your Lord knows best who is misguided from His way. And He knows best who are guided. (Surat an-Nahl; 16:125)
...You will find the people most affectionate to those who have iman are those who say, 'We are Christians.' That is because some of them are priests and monks and because they are not arrogant. (Surat al-Ma'ida; 5:82)
Only argue with the People of the Book in the kindest way - except in the case of those of them who do wrong - saying, 'We have iman in what has been sent down to us and what was sent down to you. Our God and your God are one and we submit to Him.'(Surat al-'Ankabut; 29:46)
...There is a community among the People of the Book who are upright. They recite Allah's Signs throughout the night, and they prostrate. They have iman in Allah and the Last Day, and enjoin the right and forbid the wrong, and compete in doing good. They are among the salihun. You will not be denied the reward for any good thing you do. Allah knows those who have taqwa. (Surah Al 'Imran: 3:113-115)
A well-known Rig Vedic hymn stemming from Hinduism claims that "Truth is One, though the sages know it variously.", thus proclaiming a pluralistic view of religion.
Krishna, incarnation or Avatar of Vishnu, the supreme God in Hinduism, said in the Gita: In whatever way men identify with Me, in the same way do I carry out their desires; men pursue My path, O Arjuna, in all ways. (Gita:4:11);
Krishna said: "Whatever deity or form a devotee worships, I make his faith steady. However, their wishes are only granted by Me." (Gita: 7:21-22)
Another quote in the Gita states: "O Arjuna, even those devotees who worship other lesser deities (e.g., Devas, for example) with faith, they also worship Me, but in an improper way because I am the Supreme Being. I alone am the enjoyer of all sacrificial services (Seva, Yajna) and Lord of the universe." (Gita: 9:23)
Shoghi Effendi, the head of the Bahá'í Faith in the first half of the 20th century, states: "The fundamental principle enunciated by Bahá'u'lláh, the followers of His Faith firmly believe, is that religious truth is not absolute but relative, that Divine Revelation is a continuous and progressive process, that all the great religions of the world are divine in origin, that their basic principles are in complete harmony, that their aims and purposes are one and the same, that their teachings are but facets of one truth, that their functions are complementary, that they differ only in the nonessential aspects of their doctrines, and that their missions represent successive stages in the spiritual evolution of human society." 
^Lewis, Mere Christianity (New York: Macmillan, 1967), 65. For a study of Lewis on this topic see John Sanders, No Other Name: An Investigation into the Destiny of the Unevangelized (Grand Rapids, Mich.: Wm. B. Eerdmans, 1992), 251-257.
^Wesley, "On Faith" in The Works of John Wesley, third edition volume 7 (Peabody, Mass.: Hendrickson, 1986), 197. For other texts by Wesley on the topic see John Sanders,No Other Name: An Investigation into the Destiny of the Unevangelized (Grand Rapids, Mich.: Wm. B. Eerdmans, 1992),249-251.
^Clark Pinnock, A Wideness in God's Mercy: The Finality of Jesus Christ in a World of Religions (Grand Rapids: Zondervan, 1992)
^"I Can't Play God Anymore" interview with James M. Beam, McCall's Magazine, (January 1978), pp. 154-158