I am trying to say that saving the environment will guarantee a future for future generations. Does it actually mean that or have I made mistakes?
A direct translation of 我认为保护环境将为子孙后代保证未来 will be something close to:
I believe (我认为) protecting the environment (保护环境) will guarantee a future for offsprings (将为子孙后代保证未来)
While It's understandable, a better way of saying it could be:
1) 我认为 (I believe) was not in the origin content
2) 子孙后代 vs 下一代 - (your) offsprings vs future generations (which does not necessary related to you by blood)