I've read several books of late in which the authors profess certainty about several Biblical matters that I perceive to not be completely black and white. I reference here no primary doctrines that are clearly stated in Scripture, but rather lesser issues about which sincere believers can have honest disagreement.
Although being pretty opinionated myself, I do seek to avoid bold pronouncements about matters that God seems to have left less than definitive. I happen to agree with the perspectives of the authors referenced above, but do not believe that I can categorically state "Thus saith the Lord" about the matters they address. I mean this as no criticism to the authors, who were fine and godly men, and who may have studied the issues in question more than I have (and thus believed they could be pretty emphatic). I don't think this is the case, however, and I prefer to acknowledge some Biblical latitude about the issues.
Born again believers in the Lord Jesus Christ must have both conviction and humility residing within our hearts and minds. Regardless of how much we know, we "know nothing yet as we ought to know" (I Corinthians 8:2). Certainly we can understand Scripture well enough to be confident that we walk in Truth and in expectation of consistent godliness. However, a cocksure attitude is far different than the assurance produced by regular reading of Scripture and the Holy Spirit's humility-producing illumination. The more we know, the more we realize we need to know as the infinite light of God's Word both enlightens and exposes our darkness.
Believers thank God for those clearly stated truths of the Bible that we boldly and humbly affirm without equivocation. We also thank Him for those issues not so completely delineated in Scripture. About such matters we can have honest and friendly discussion, and even disagreement. Most importantly, all Truth ultimately leads us to bow together in the enlightened faith that looks to God alone as the knower of Himself and His truth. A far more authoritative and powerful confession proceeds forth from the holy bond of conviction and humility that sincere consideration of the Bible produces. Affirmation of what we know and acknowledgement of what we don't know unite to lead others to trust not us, but the One whose "understanding is infinite" (Psalm 147:5).
"The secret things belong unto the LORD our God: but those things which are revealed belong unto us and to our children for ever, that we may do all the words of this law."