"To some who were confident of their own righteousness and looked down on everybody else, Jesus told this parable: "Two men went up to the temple to pray, one a Pharisee and the other a tax collector. The Pharisee stood up and prayed about himself: 'God, I thank you that I am not like other men—robbers, evildoers, adulterers—or even like this tax collector. I fast twice a week and give a tenth of all I get.'
"But the tax collector stood at a distance. He would not even look up to heaven, but beat his breast and said, 'God, have mercy on me, a sinner.'
"I tell you that this man, rather than the other, went home justified before God. For everyone who exalts himself will be humbled, and he who humbles himself will be exalted."
Most Christians would claim to identify with the tax collector and thank God that we are saved by Grace and not by our own efforts. Yet once we are saved we are often subtly drawn back to effort-based salvation. In my own experience that has manifested itself in feeling confident to approach God because certain "big" sins have not been evident recently. This is so wrong.
1 John 1:8-10 says "If we claim to be without sin, we deceive ourselves and the truth is not in us. If we confess our sins, he is faithful and just and will forgive us our sins and purify us from all unrighteousness. If we claim we have not sinned, we make him out to be a liar and his word has no place in our lives."
Its important to distinguish between our relationship and our fellowship with God:
We have been saved by Grace and are are children of God. He has "set his seal of ownership on us, and put his Spirit in our hearts as a deposit, guaranteeing what is to come" - 2 Cor 1:22. The relationship is secure.
What isn't secure however is our fellowship with Him. If we sin, we do not walk in the light, and have no fellowship with Him. No fellowship means unanswered prayers, no guidance, no victory, no joy and no peace.
So approach His throne of Grace with confidence because of your relationship with Him - you are his child and he loves you unconditionally, but confess your sins so that nothing will obscure your fellowship and walk with Him.