Possible meanings are 1) "God who confirms our relationship with each other because we are in Christ" or 2) "God who confirms both our and your relationship with Christ."
Possible meanings are 1) "he sent us to preach the gospel" or 2) "he chose us to be his people."
# he set his seal on us
Paul speaks of God showing that we belong to him as if God had put a mark on us as a sign that we belong to him. AT: "he has put his mark of ownership on us" or "he has shown that we belong to him" (See: [[rc://en/ta/man/translate/figs-metaphor]])
# gave us the Spirit in our hearts
Here the word "hearts" refers to the innermost part of a person. AT: "gave us the Spirit to live within each of us" (See: [[rc://en/ta/man/translate/figs-synecdoche]])