But we ought always to give thanks to God for you, brothers beloved by the Lord, because God chose you as the firstfruits to be saved, through sanctification by the Spirit and belief in the truth. (2 Thess. 2:13)
We have previously seen that true biblical faith (as opposed to mystical "faith") consists in trusting wholeheartedly in Christ and His Word as found in the Scriptures. On the other hand, true knowledge is possible despite our fallen-ness, and knowledge of the Gospel is essential for salvation. The question is then raised, what is the relationship between faith and truth?
The secular idea of faith is the existential or Kierkegaardian one, whereby knowledge is not only not related to faith but inimical to it. Faith is considered a "blind leap" whereby we believe regardless of the facts (and possibly even contrary to it — "leap of faith"). If however biblical faith is to submit to God and His Word as we have previously seen, then such a conception of faith is not only suspect, but even dangerous to believers. In point of fact, the two concepts of faith (Kierkegaardian and biblical) are antithetical to each other, such that embrace of the one would exclude the embrace of the other.
We have already established that some knowledge of Christianity, i.e. the Gospel, is necessary unto salvation. Yet, we are being saved by faith alone, not faith plus knowledge. A contradiction seems inevitable, until we peek into the mechanics of faith itself.
Modern Reformed theologians define faith as being made up of knowledge (cognitio), assent (assentia) and trust (fiducia). While disputes over the third element remain (mainly between Van Tillians and Clarkians, since historically Reformed theologians have not been so clear cut over the exact ontology of faith — See Herman Witsius' discussion on the subject for example in his Economy of the Covenants III.7.8 - III.7.24, or John Calvin in his Institutes of the Christian Religion, III.2), the focus here would be on the first two parts of faith in light of Scripture,
In 2 Thess. 2:13, we are told that we are "to be saved"... "through... belief in the truth" (πιστει αληθεια). If our salvation comes through the instrument of believing the truth, then faith if it is alone must include the element of "belief in the truth". In fact, since "faith" and "belief" have essentially the same Greek root (πιστις), we see that "truth" is the object of faith. Knowing however that Christ is the object of our faith (ie. Acts 20:21) and that Christ is the incarnated Logos (Jn. 1:1,14), we see therefore that Christ and truth are linked (Jn. 17:17). To belief in Christ is to believe in truth, and to believe in truth is to believe in Christ; to believe in Christ is to trust His Word in Scripture (Jn. 1:1, 2 Tim. 3:16, 2 Peter 1:21) as truth, and vice versa.
Since such is the case, faith includes a preliminary belief in the truths of the Gospel, and a progressive growth in knowledge and assent to the truths of the Christian faith. Epistemologically, Christ can only be known through His Word the Scriptures, and therefore to claim to know God more yet not to know more truths regarding Him is a self-contradiction in terns. Faith and truth are therefore not existential. While certainly they are more than intellectual, they are not less than that.
Truth therefore is a fruit of true faith. Without faith it impossible to please God (Heb. 11:6). Knowing the correlation between faith and belief in the truth, we can also say that "without understanding and assenting [and trusting] the truth it is impossible to please God". Mysticism in whatever form is therefore not of God. Even if supposed 'great men of God' (i.e. the Desert Fathers) taught and practiced them, such practices are not of faith.