Of course, that's what believing means. For there is nothing else to believe in that will lead one into salvation, or to receive approbation from God. Christ is the only way under the sun, to gain God's acceptance, for all peoples on the earth.
What you say is true, it has always been by grace through faith, and all based on the redemption in Christ.
The twelve apostles and all the disciples were not in the body of Christ. They will be raised to enter the kingdom on earth. The twelve will rule over the twelve tribes on earth (Matthew 19:28).
The body of Christ will be in heavenly places. The body of Christ was not revealed until it was made known by Paul.
All things will be gathered together in Christ (Ephesians 1:10).