"solemn pledge" is correct and usable in written English.
You can use it to express a promise or commitment that is serious and sincere. For example: I give you my solemn pledge that I will never break your trust.
In return we give them a solemn pledge: that we shall keep their role secret".
This is a solemn pledge that they won't, among other things, lie, cheat and watch porn.
They criticized Mr. Obama for the delay, saying it breaks a solemn pledge to immigrants.
In return, we give them a solemn pledge: that we shall keep their role secret.
Representative Jerrold Nadler of Manhattan said, "The bill before this House welshes on that solemn pledge".
Kofi Annan: I myself have made that point, that member states made a solemn pledge to protect.
Leaders issue profound words of regret, accompanied by solemn pledges that such calamities cannot be allowed to happen again.
Murdoch was able to acquire both publications in 1981 only because he began making solemn pledges that he would maintain the tradition of independence.
But here, for those wondering about her ability to flout any of her own government's solemn pledges, is a whopper that has left millions of UK citizens in the lurch.
Nor did he bring it up at a fund-raiser here, where he used humor, pointed critiques and solemn pledges about his dedication to protecting the nation to draw distinctions between himself and Mr. Kerry, the likely Democratic nominee.
The international development secretary has shadowed the post since 2005 and watched world leaders make solemn pledges of financial support at the G8 Gleneagles summit to help deliver the UN's eight Millennium Development Goals.
