We come before the Lord in trust. We have to do this because without it, nothing we do in worship will prevail with Him. We trust Him, we have faith in Him, because this is the only way that creatures can receive blessing from Him.
With His blessing, preaching against sin brings conviction and change. Without it, such preaching brings resentment and continued sinning. With His blessing, the singing is rich, and grateful, and overflowing. Without it, the singing is muttered and resentful. With His blessing, the prayers are urgent, anointed, honoring to Him. Without it, the prayers are wooden and stilted. With His blessing, the Supper of the Lord nourishes in Christ, and grows us up into Him. Without His blessing, this Supper is a winnowing fan that blows chaff away from His presence.
And so we are to come in faith. We assemble in faith, we hear the call to worship in faith, we confess our sins in faith, we sing in faith, we hear the Word read in faith, we hear the Word preached in faith, and we eat and drink in faith. Without faith, it is impossible to please Him.
The only alternative to this faith is obviously a lack of it—unbelief. But this unbelief rarely parades itself in naked form. Unbelief always seeks various rags and scraps to clothe itself. These rags and scraps are often made of resentment, grievances, bitterness and more. Guard your hearts therefore. You are coming to worship the Lord. Do so in faith, from first to last.