Christianity, Islam, and Buddhism stand among the largest religions in the world.  Taken at face value, it is easy to see that the faith systems share many commonalities. However, it is in assessing the central questions common to all three that their critical differences become apparent.
It is said that all worldviews answer similar questions, such as, “Is there a higher power in the universe?… Where did our world come from?… What is the purpose of man?”  Questions such as these have practical implications for individual adherents and speak to another, related question, “How shall I live?”  Christianity, Islam, and Buddhism largely agree on this final point; many moral and ethical teachings are common to each faith system and it is easy to understand why someone might ask, “If moral outcomes are the same across faiths is there really a meaningful difference between the religions.?”  However, it is answering the first three questions listed here that one can see the vast divide that exists between each faith. Taken at face value, it seems faith systems share many commonalities. But when assessing the central questions, their critical differences become apparent. Click To Tweet
Christianity and Islam answer the first question, “Is there a higher power in the universe?” in the affirmative. Both Christianity and Islam believe a god exists and that God is a personal being. Buddhism is largely silent on the issue; it could be said that Buddhism is atheistic or agnostic, perhaps even deistic in its view.  However, a personal God who has desires for His creation and with whom humanity could be in relationship (such as in Christianity and Islam) is incompatible with Buddhism.
Thus, the first question one could ask of all three faiths is this: what evidence, if any, exists to indicate a God exists or what that God would be like? Both Christians and Muslims have historically offered variations of the Kalam cosmological argument for the existence of God  This line of reasoning indicates the existence of a cause for the universe and, when one considers the properties such a cause would have, suggests that the cause is a personal God. Further arguments, such as the moral argument, also indicate that a God exists and is personal.  Not only does the moral argument point to a personal God, but points to one which has desires for people to carry out and which would have a practical input on the daily lives of believers. These sorts of arguments strongly favor the Christian or Islamic views; Buddhism is unlikely to be true based on such arguments.
The Kalam argument also speaks to the second question, “Where did our world come from?” Christian scripture indicates God exists and created everything from nothing. Genesis 1:1 reads, “In the beginning God created the heavens and the earth.” Islam, too, teaches a similar origin story in the Quran, “Allah created the heavens and the earth, and all that is between them, in six days” (7:54). Buddhism, however, does not address origins, simply teaching that reality exists and must be accepted. 
It may seem, based on this brief summary so far, that Christianity and Islam are on the same page. However, it is in the third question that they widely differ. Many may see the question, “How shall I live?” as referring to practical, physical behaviors. It is in outward behavior that Islam is certainly interested. Islam teaches that Muslims must adhere to strict moral teachings and that they will be judged according to their ability to keep God’s commandments.  Christianity and Buddhism, however, take different tacts. Buddhism sees the principle problem as being not in the sins of individuals (indeed, Buddhism teaches the individual does not truly exist)  but in the rejection of want and the reaching of a state of nirvana (itself a difficult concept to define). Christians agree with Muslims that it is human sin that is the central problem, but reject the idea that doing good deeds could overcome sin. Instead, Christians believe it is through the grace of Jesus Christ (himself God) that one is forgiven; trust in Jesus, not one’s own works, is what is needed. However, Christians believe it is in response to God’s mercy that Christians should do good deeds and show mercy to others. 
However, there is another, singular question which I believe could be asked to determine which, if any, of the three faiths discussed here is true:
“Who was Jesus?”
Jesus is explicitly mentioned in Christianity and Islam, although the specific details differ. In Christianity, Jesus is God incarnate, who has come to earth to die by crucifixion so that the sins of the world may be forgiven.  In Islam, Jesus is a prophet of God, but is not himself divine. Further, Jesus is not crucified and does not die for the sins of the world.  Buddhism began long prior to the time of Jesus and as a result the early teachings do not mention Jesus; however, some modern Buddhists have described Jesus as enlightened (although certainly not God incarnate). 
It is important not only to understand what each faith says about Jesus, but what can be known of Jesus from a historical perspective. First, it is important to note that the earliest literature about Jesus is Christian in nature. The earliest portions of the New Testament were written within at least a couple decades of Jesus’ death  and evidence suggests the earliest beliefs about Jesus originated within only a few years of his lifetime.  Muslim claims about Jesus, however, originate hundreds years after Jesus. Further, non-Christian historians living close to the time of Jesus affirm that he lived, died by crucifixion, and that his followers believed him to be divine, consistent with Christian claims.  Not only that, but the earliest record of Jesus’ teachings depict his making exclusive religious claims such as, “I am the way, and the truth, and the life; no one comes to the Father except through Me.” (John 14:6) This would mean Jesus himself excluded Buddhism, which predated him, from being a possible true faith. Late Buddhist attempts to include Jesus in their worldview must reject the historical Jesus. As a result, the Christian faith makes the most accurate and reliable claims about Jesus. Further, the evidence for Jesus’ resurrection is compelling.  As a result, both Islam and Buddhism make claims which are in direct conflict with the truth who Jesus was. Christianity alone passes this test.
There is much to say when it comes to these faith systems, certainly much more than could be covered by this brief review. However, it is important to note that several questions are common to all three views. By focusing on questions common to these faiths, it is easy to, by way of comparison and contrast, determine which is most likely to be true: namely, Christianity.
 World Population Review, “Religion by Country 2021,” accessed January 31, 2022. https://worldpopulationreview.com/country-rankings/religion-by-country.
 Herb Suereth, “Your Worldview and Why it Matters: 3 Questions to Determine Worldview,” Veritas Academy, last modified December 19, 2016. https://www.veritasacademy.com/headmasters-blog/your-worldview-why-it-matters-3-questions-to-determine-worldview.
 Steve Leashomb, “The Four Biggest Questions Your Worldview Must Answer,” Midway Baptist NC, last modified June 4, 2020. https://www.midwaybaptistnc.org/blog/the-four-biggest-questions-your-worldview-must-answer
 A similar question is addressed in Douglas Groothuis, “Jesus and Buddha: Similarities and Differences,” February 3, 2014, YouTube video, 1:30:16, https://youtu.be/0lUB4P8XbqE.
 Irving Hexam, Encountering World Religions (Grand Rapids: Zondervan, 2019), 81-86.
 William Lane Craig, “The Kalam Cosmological Argument,” Reasonable Faith, accessed January 31, 2022. https://www.reasonablefaith.org/writings/popular-writings/existence-nature-of-god/the-kalam-cosmological-argument.
 Richard G. Howe, “What Are the Classical Proofs for God’s Existence?” In The Harvest Handbook of Apologetics, ed. Joseph M. Holden (Eugene: Harvest House Publishers, 2018), 86-87.
 Groothuis, “Jesus and Buddha: Similarities and Differences.”
 Hexam, Encountering World Religions, 171.
 Douglas R. Groothuis, “Jesus and Buddha: Two Masters or One?” Christian Research Institute, last modified June 9, 2009. https://www.equip.org/article/jesus-and-buddha/.
 See Ephesians 2 and James 2.
 1 Corinthians 15:3-8 reads, “For what I received I passed on to you as of first importance[a]: that Christ died for our sins according to the Scriptures, hat he was buried, that he was raised on the third day according to the Scriptures, and that he appeared to Cephas, and then to the Twelve. After that, he appeared to more than five hundred of the brothers and sisters at the same time, most of whom are still living, though some have fallen asleep. Then he appeared to James, then to all the apostles, and last of all he appeared to me also.”
 Quran 4:157 reads, “That they said (in boast), “We killed Christ Jesus the son of Mary, the Messenger of Allah”;- but they killed him not, nor crucified him, but so it was made to appear to them, and those who differ therein are full of doubts, with no (certain) knowledge, but only conjecture to follow, for of a surety they killed him not.”
 Nyogen Senzaki, “Not Far From Buddhahood,” in 101 Zen Stories (London: Rider, 1919).
 Robert Wall. New Interpreter’s Bible Vol. X (Nashville: Abingdon Press, 2002), 373.
 Richard Carrier, “Dating the Corinthian Creed,” RichardCarrier.org, last modified August 10, 2016. https://www.richardcarrier.info/archives/11069.
 Gary R. Habermas, The Historical Jesus: Ancient Evidence for the Life of Christ (Joplin: College Press Publishing Company, 1996), 187-228.
 Although to extensive to go into here, see the minimal facts argument in Gary R. Habermas and Michael R. Licona, The Case for the Resurrection of Jesus (Grand Rapids: Kregel Publications, 2004), 43-77.