This issue isn't really anything to do with Christianity, although it's an issue which Christians sometimes take sides on.
When the State of Israel was created in 1947-48 Jerusalem was given a special status, not a part of Israel proper: "The City of Jerusalem shall be established as a corpus separatum under a special international regime and shall be administered by the United Nations." The Israeli capital was established as Tel Aviv
Israel took effective control of West Jerusalem in 1948 and the rest of it in 1967, and has retained it ever since, declaring it to be their capital. Many nations, including until now the US, retain their diplomatic missions in Tel Aviv and consider it to be the capital, avoiding the implied recognition of the legitimacy of Israel's control over Jerusalem.