I disagree with you that all Christians are in that covenant relationship with God Scott. Why? One has to simply read who he made the covenant with, and why. Remember there were millions of God's people there when Jesus made that covenant, and he only did it with his faithful 11 apostles. Only 144k are in that covenant relationship, bought out from among mankind to serve as Kings and Priests under King Jesus in the coming Kingdom of God on earth, however their residence is in heaven.Here is one,
Jesus is mediator for the 144,000 alone.
"Likewise, the Greater Moses, Jesus Christ, is not the Mediator between Jehovah God and all mankind. He is the Mediator between his heavenly Father, Jehovah God, and the nation of spiritual Israel, which is limited to only 144,000 members." Worldwide Security Under the "Prince of Peace" (1986) pp.10-11
"The "great crowd" of "other sheep" that is forming today is not in that new covenant. However, by their associating with the "little flock" of those yet in that covenant they come under benefits that flow from that new covenant." Watchtower 1979 Apr 1 p.31
Scripture though says all believers are members of the new covenant established in Christ's blood, as they are saved by believing, not just 144,000 are members of the New Covenant. If you are outside that covenant, you are not saved, meaning you will be destroyed in the Lake of Fire at the end of the age.
Hebrews 12:24
to Jesus the Mediator of the new covenant, and to the blood of sprinkling that speaks better things than that of Abel.
1 Timothy 2
3 For this is good and acceptable in the sight of God our Savior,
4 who desires all men to be saved and to come to the knowledge of the truth.
5 For there is one God and one Mediator between God and men, the Man Christ Jesus,
6 who gave Himself a ransom for all, to be testified in due time,
Hebrews 9
14 how much more shall the blood of Christ, who through the eternal Spirit offered Himself without [a]spot to God, cleanse your conscience from dead works to serve the living God? 15 And for this reason He is the Mediator of the new covenant, by means of death, for the redemption of the transgressions under the first covenant, that those who are called may receive the promise of the eternal inheritance.
Acts 2:21
And it shall come to pass That whoever calls on the name of the Lord Shall be saved.’
Acts 16:31
So they said, “Believe on the Lord Jesus Christ, and you will be saved, you and your household.”
Romans 10:9
that if you confess with your mouth the Lord Jesus and believe in your heart that God has raised Him from the dead, you will be saved.
You are very correct that Jesus is the mediator between all humans and God, no one else is! Every single individual that prays approaches God in Jesus' name, if they do it correctly anyway. I hope you understand that calling on the name of the Lord, is referring to God not Jesus, it was quoted from the Hebrew Scriptures at Joel 2:32. I hope you further understand that more than belief in Jesus is required for salvation. It would be nice if Romans 10:9 was the only verse in the Bible, but unfortunately there is much more involved for salvation Scott.