Psalm 103:19 affirms that the sovereignty of the living God rules over all. Now, as it is that God is infinite, so His sovereignty is absolute, for, His attributes are but expressions of who He is in His essence. Anything less, more, or different would not describe the one and only living God. To portray Him in any terms or fashion that do not accurately reflect His biblical presentation is to be guilty of idolatry. Indeed, one of the most basic purposes God had for breathing out His Word, providentially preserving it, and providing understanding of His truth by the regenerating work of His infallible Spirit, is to be worshiped as He truly is. No, we are not to concoct our own concept(s) of who He is and what He requires/desires of us... we are not to base our lives on speculation, but revelation. To the extent that we are ignorant of revelation (the Scriptures), is the extent to which we will use our sanctified imagination to speculate what seems right and accurate.
Major problems with speculation, however, are that (a) it lacks God's authority, (b) our hearts are more deceitful than all else, and desperately sick [Jeremiah 17:9], and (c) it is inevitably idolatrous, for He Himself declares, "My thoughts are not your thoughts, Neither are your ways My ways. For as the heavens are higher than the earth, So are My ways higher than Your ways, And My thoughts than your thoughts," [Isaiah 55:8, 9].
As for (a), speculation in the spiritual realm doesn't have God's assurances of power, as does His Word: "For as the rain and the snow come down from heaven, And do not return there without watering the earth, And making it bear and sprout, And furnishing seed to the sower and bread to the eater; So shall My word be which goes forth from My mouth; It shall not return to Me empty, Without accomplishing what I desire, And without succeeding in the matter for which I sent it," [Isaiah 55:10,11]. "For the Word of God is living and active and sharper than any two-edged sword, and piercing as far as the division of soul and spirit, of both joints and marrow, and able to judge the thoughts and intentions of the heart," [Hebrews 4:12]. Such promises as these (and many more) are given by the unchanging (Malachi 3:6a) living God, who does not lie (Numbers 23:19), and is utterly faithful (James 1:17). Our opinions are exactly that: opinions... oscillating as we take in new knowledge, and so cannot come close to the authority of the omniscient God of the Word.
Concerning (b), if one ignores or refuses the Word that God has inspired, preserved and revealed, he is left to depend upon his heart's imagination to answer such questions as "What is God like?" "What happens to me after death?" and "Is Jesus really the only way to heaven?" Man lays great stock in his heart's ability to judge life correctly: As Shakespeare put it,"To thy own self be true, and it must follow, as the night the day, thou canst not then be false to any man." Sounds reasonable... trouble is, it won't do. Contrary to the unsaved person's insistence, the lost one is spiritually blind (II Corinthians 4:3,4) and unable to know his own heart, much less be true to it. As Proverb 14:12 states, "There is a way which seems right to a man, but its end is the way of death." Only through salvation (had only through the Lord Jesus Christ) does one have spiritual discernment.
Regarding (c), any way that we try to relate to the living God other than how He is presented in Scripture, is idolatry. Indeed, what comes to mind when we think of God is a spiritual barometer, revealing whether or not He has the same attributes in our thoughts as He has in the Bible. Operating apart from revelation, man's speculations are always skewed, dominated by emotion and logic. Since "every man's way is right in his own eyes," (Proverb 21:2a), he/she will envision God through feelings and suppositions...many based in childhood experiences, as well as traditions assumed to be true (one such entrenched assumption is "God helps those who help themselves"). Many even believe this saying is found in Scripture, it is so pervasively accepted. It isn't. And it helps perpetuate the myth that our works are the means by which we gain God's aid & acceptance. No, only by revelation do we learn the ways of grace.
May God grant a deeper desire to learn Him as He is...and seek His face through the revealed Word He has given so graciously.