The question of whether faith can be proven by evidence or not is addressed by Thomas Aquinas in his Summa Theologica (1.1.8). Thomas first lists the common objections:
1. Christianity is built on faith, not proven by arguments.
2. Argument is either from authority, which is weak in proofs, or from reason, which is not built from faith.
Thomas then lists the following in his response:
- No field of study argues for its first principles, but rather argues from its first principles to go on to prove something else. For example, the Apostle Paul starts with the resurrection of Jesus and argues to the general resurrection. Therefore no field of study attempts to prove the basic, first principles upon which it is founded, but instead uses them to argue to conclusions.
- Sacred scripture can make an argument only if someone agrees that some of its principles are true. If someone disputes everything in the scriptures, it can make no argument.
- We can argue with heretics from scripture because they agree on some things of divine revelation. Likewise we can argue with people who deny only one or two things of theology.
- A falsehood cannot be demonstrated. Since scripture is true, then proofs brought against scripture cannot be demonstrated, but can be answered.
Therefore when we are dealing with those who agree that some things in scripture are true, we can work from scripture to correct them. But for those who dispute everything in scripture, we have nothing in scripture to use as a basis for proof to them. For these people, their arguments are false and can be refuted from wisdom alone.
Thus Christianity can start with faith and argue to other truths. But no argument in any field can prove a first principle, and it is absurd to expect this in theology.
Theology proper begins with statements made by authority (scriptures), so to argue from authority is most proper, since its principles are obtained by revelation. We argue not to prove the things revealed by authority (faith), but rather to make clear the things that are put forward in reveled truth. Doctrine is built upon first principles of theology, and higher doctrines are built upon lower or more simpler doctrines. Christianity can then start with faith (revealed by authority) and argue to other truths.
Thomas had already shown in a previous question that doctrine can be investigated and God is the subject of this investigation.