How can we know if something is true?
On one hand, we can presuppose that something is true without requiring evidence, such as the existence of God. This is essentially faith, as described in Hebrews 11:1: "Now faith is the assurance of things hoped for, the conviction of things not seen."
For instance, one might presuppose that logic, mathematics, and other preconditions for intelligibility are only possible if God exists. Since these conditions exist, God must exist. However, without evidence, we could just as easily assume the opposite: perhaps these preconditions are only possible if God does not exist. Based on presuppositions alone, there's no way to determine which view is correct.
Therefore, we must turn to evidence—what can be observed, measured, and tested (evidentialism). Romans 1:20 supports this idea: "For since the creation of the world God's invisible qualities—his eternal power and divine nature—have been clearly seen, being understood from what has been made."
However, evidentialism also relies on unprovable assumptions. To trust evidence, we must assume that our senses, methods of testing, and natural laws are reliable and consistent—assumptions that cannot be proven without circular reasoning.
In short, both approaches are circular. Presuppositionalism assumes God's existence to make sense of the world, while evidentialism assumes the reliability of our senses and natural laws. Both assumptions are unprovable without employing circular reasoning.
This doesn't mean either approach is wrong, only that both are bound by the same limitation. This is why agnosticism on truth claims is the default position.