Holy is the Lord
When we do theology (when we think about and try to understand God), we're at a disadvantage: God is eternal and infinite; we are mortal and finite. There is no way for the mortal and finite to comprehend the eternal and infinite.
In addition to that, we suffer from the noetic effects of the fall: this means that our sinfulness distorts our perspective. So our disadvantage in trying to understand God is huge. This is why we depend on God's revelation to us. In his Word to us - the Bible - God reveals many things about himself: He is gracious, loving, merciful, kind, good and righteous. These are the things we like about God.
But in addition to these positive traits, there is one 'master' trait that informs all the others: holiness. God is holy in such a way that everything about him is holy: his grace, love, mercy and kindness are all holy.
In the Old Testament, the Hebrew word for holy is קָדוֹש - quadosh. This means that God is both separate and pure: God is creator, completely distinct from creation; God is absolutely pure and lacks any moral defilement.
This is difficult for us to grasp. First, everything we know is created or derivative of something created; God, however, is uncreated. God is both infinite and eternal; we are finate and mortal. And it is impossible for the finite and eternal to fully grasp the infinite and eternal.
Additionally, God is absolutely pure; in him, there is no moral defilement at all. Again, everything we know has some degree of moral mixture; no one is completely pure in their motivations and thoughts.
Without revelation, without God revealing himself to us in Christ and in his word, we would not know who he is or what he is like. But least we think the revelation God is a bundle of joy, not only do we bump into the God is gracious, but also the God is who is holy.
And this is our problem. And it was Isaiah's problem, because when he saw the Lord (Isaiah 6:1), when he heard the God was holy (Isaiah 6:3), his response was "Woe is me! For I am man of unclean lips, and I live among a people of unclean lips' (Isaiah 6:5). That is, when we encounter the living God, it exposes our sinfulness. So here is our problem:
God is more holy than we can imagine; we are are more sinful than we will admit.
If left on his own, Isaiah, would have been in trouble, but at that moment, a seraphim touched his mouth with a coal from the altar and said, '“Behold, this has touched your lips; your guilt is taken away, and your sin atoned for.” God did for Isaiah what he could not do for himself.
The author of Hebrews describes a similar, but far superior, 'touching': Therefore, we have confidence to enter the holy places by the blood of Jesus, by the new and living way that he opened for us (Hebrews 10:19). And that brings us to the big point:
We can’t give ourselves access to God’s presence; only God can give us access to his presence. And he does this through the blood of Christ.