Ligon Duncan, a signer of the Manhattan Declaration and president of The Alliance of Confessing Evangelicals, explains that they have received a number of requests concerning the Manhattan Declaration and why some have signed it with leaders from Roman Catholic and Eastern Orthodox traditions. He responded with a statement that has been posted at the Ref21 blog. He concludes with these words:
The issue boils down to a matter of judgment, not a disagreement in principle, between those Council members who signed and didn’t sign. The non-signers believe that the content of the document and the associations of the primary authors imply an ECT-like confusion about the Gospel. The signers believe that the explicit assertions and emphasis of the documents relate only to areas of principled social-ethical agreement between evangelicals and non-evangelicals. Further, they believe that it is important for individuals from the major quadrants of the historic Christian tradition to speak on these pressing matters in solidarity.
The Council members have had good, robust discussions on these things among ourselves about this whole matter. We continue to love and respect one another, and we all want to continue to serve and work with one another. The bonds of our fellowship are unbroken. Our commitment to the mission of the Alliance is unchanged. Our unity in the Gospel, and in the great solas of the Reformation is stronger than ever.
You can read his whole statement here.