You're not coming across as strong, per se, but with a bad hermeneutic. As I noted, Christ did not quote an *explicit* Scripture to silence the Sadducees but reasoned from the Scriptures. It's not going "beyond" the Scriptures to reason from their implications. Proof-texting has its place but your theology is incredibly stilted and naive if you limit yourself to it.I want to avoid coming onto too strong, because if the Scripture does not answer the question explicitly, Id on't want to go beyond the Scripture. That being said, I don't think you are addressing the actual points being made in the above. If the issue was simply settled by creeds we would have no need to simply ask the question. Being that the question was asked, and ideally even if a creed is correct it is substantiated by the Scripture, I think the question should be answered by the Scripture.
Covenant theology arises from the Scriptures both exegetically as well as by GNC. The necessary consequence from many Scriptures is that children of believers are constituted as holy by the Lord. Promises are made concerning the salvation of believers and their children. The good consequence is that believing parents ought not to doubt the salvation of their children but simply believe that God promised to be God to them in His Promise. You may dismiss David's confidence but you have not exegetically established that the child is not with the Lord but only offered an opinion from what I consider bad GNC based on the nature of God's Promises.