What exactly constitutes a "religious test" as defined in the US Constitution?

Question

Article 6 of the US Constitution states that no "religious test" can be required for any office or public trust.

The Senators and Representatives before mentioned, and the Members of the several State Legislatures, and all executive and judicial Officers, both of the United States and of the several States, shall be bound by Oath or Affirmation, to support this Constitution; but no religious test shall ever be required as a qualification to any office or public trust under the United States.

What exactly is a "religious test" in this context? Is it adherence to a religion (e.g. "you must/must not be Protestant")? Is it having a religious belief (e.g. "you must/must not have religious grounds for abortion immoral")? Or something else?

To give a concrete example, this article claims that Democratic senators are creating a religious test by indicating that they are not wanting to approve a judicial nomination who is a member of the Knights of Columbus (a Catholic volunteer organization that is pro-life). Assuming that this an accurate assessment of their motivation, would that count as a religious test?

Answer 1

Religious Tests Prior to the Constitution

Prior to the Constitution, 9 out of 13 states had religious requirements for officeholders. For example, Georgia's constitution explicitly required legislators to be Protestant:

The representatives shall be . . . of the Protestent religion. . . .

Others, such as Delaware, required legislators to declare their religious beliefs:

Every person who shall be chosen a member of either house, or appointed to any office or place of trust, before taking his seat, or entering upon the execution of his office, shall take the following oath, or affirmation, if conscientiously scrupulous of taking an oath, to wit: “I, do profess faith in God the Father, and in Jesus Christ His only Son, and in the Holy Ghost, one God, blessed for evermore; and I do acknowledge the holy scriptures of the Old and New Testament to be given by divine inspiration.”

These are what the Constitution means by "religious tests". They are formal religious requirements for holding office.

Answer 2

Religious Tests for State Offices were struck down as unconstitutional in the 1961 case Torcaso v. Watkins, which ruled that Maryland's provision on non-atheists serving in office violated an atheist's right to religious freedom. Since then many suits were filed that removed such tests at the state level. It should not be pointed out that this ruling does not rely on the no religious test, which strictly bars this practice in the Federal Government but not the States, but rather on 1st amendment protections of free religious practice and the 14th Amendment. SCOTUS did not rule on the Religious Test Clause because the decision was made off another basis entirely (SCOTUS typically will not decide a second matter if a first one satisfies a majority of the court.).

With regards to @Thunderforge's question, this has come up because an unusual number of SCOTUS judges are Catholics, despite the US being one of the few Christian Majority nations with a Catholic Minority and there have been statements by politicians that have made mention of scrutiny of future Catholic nominees to the Supreme Court. While these statements may be skirting a line, the only other ruling on a Religious Test Clause Matter was the 1867 Ex parte Garland decision.

In Ex parte Garland, SCOTUS held that the United States Federal Government could not be required to take any loyalty oaths or any oaths to serve anything that was not specifically The United States Constitution. It would stand to reason that some politicians not want a member of a particular religion to fill a particular office is constitutional, so long as the potential office holder is not made to swear an oath to their own religion or to not join the particular religion... nothing but to protect and defend the Constitution.