It is not always easy to distinguish between a rational and a spiritual understanding of Scripture. A man may come to the right interpretation of a portion of Scripture without having saving faith and the inner illumination of the Holy Spirit. He can have knowledge and assent without trust. This is a sobering and sometimes frightening fact with which we are faced. We are called to examine ourselves to know whether we are in the faith or not. This does not mean that we can never be assured that we have saving faith, or that we should doubt whether we do  every time we sin. When we sin we're told to go again to our Father in heaven in godly sorrow and cast ourselves on His mercy in Christ, who stands as our righteous advocate before the Father (1 John 1:8-2:1). It does mean, however, that we must plead with God to "open our eyes to see wonderful things" in His word (Ps. 119:18). We should not be content to merely sharpen our minds in getting the right interpretation of Scripture. We must become satisfied with gaining intellectual attainments in interpretive principles. Jonathan Edwards offered the following explanation of the difference between being able to righty interpret the Scriptures and rightly understand them with spiritual eyes:

Hence it appears, that the spiritual understanding of the Scripture, don't consist in opening to the mind the mystical meaning of the Scripture, in its parables, types and allegories; for this is only a doctrinal explication of the Scripture. He that explains what is meant by the stony ground, and the seed's springing up suddenly, and quickly withering away, only explains what propositions or doctrines are taught in it. So he that explains what is typified by Jacob's ladder, and the angels of God ascending and descending on it, or what was typified by Joshua's leading Israel through Jordan, only shows what propositions are hid in these passages. And many men can explain these types, who have no spiritual knowledge. 'Tis possible that a man might know how to interpret all the types, parables, enigmas, and allegories in the Bible, and not have one beam of spiritual light in his mind; because he mayn't have the least degree of that spiritual sense of the holy beauty of divine things which has been spoken of, and may see nothing of this kind of glory in anything contained in any of these mysteries, or any other part of the Scripture. 'Tis plain, by what the Apostle says, that a man might understand all such mysteries, and have no saving grace; I Corinthians 13:2, "And though I have the gift of prophecy, and understand all mysteries, and all knowledge, and have not charity, it profiteth me nothing." They therefore are very foolish who are exalted in an opinion of their own spiritual attainments, from notions that come into their minds, of the mystical meaning of these and those passages of Scripture, as though it was a spiritual understanding of these passages, immediately given 'em by the Spirit of God, and hence have their affections highly raised: and what has been said shows the vanity of such affections.

From what has been said, it is also evident, that it is not spiritual knowledge, for persons to be informed of their duty, by having it immediately suggested to their minds, that such and such outward actions or deeds are the will of God. If we suppose that it is truly God's manner thus to signify his will to his people, by immediate inward suggestions, such suggestions have nothing of the nature of spiritual light. Such kind of knowledge would only be one kind of doctrinal knowledge; a proposition concerning the will of God, is as properly a doctrine of religion, as a proposition concerning the nature of God, or a work of God: and an having either of these kinds of propositions, or any other proposition, declared to a man, either by speech, or inward suggestion, differs vastly from an having the holy beauty of divine things manifested to the soul, wherein spiritual knowledge does most essentially consist. Thus there was no spiritual light in Balaam; though he had the will of God immediately suggested to him by the Spirit of God from time to time, concerning the way that he should go, and what he should do and say.1

1. Jonathan Edwards Religious Affections (WJE Online Vol. 2) , Ed. Paul Ramsey p. 279