Are there different levels for non-Christians in heaven? What about those who may have thought of Christianity and lived a good righteous life, but for some reason(s) refused to become baptized?
Our Lord Jesus Christ answered, "Most assuredly, I say to you, unless one is born of water and the Spirit, he cannot enter the kingdom of God" (John 3:5).
For those who live as "good" people while refusing to believe and be baptized they are refusing Salvation. Our Lord Jesus Christ said to Nicodemus "He who believes in Him (Christ) is not condemned; but he who does not believe is condemned already, because he has not believed in the name of the only begotten Son of God" (John 3:18). The non-Christians are non-believers; so no matter how good their works are, they have been already condemned. If "good" people can enter the kingdom of God without believing in our Lord Jeisus Christ, that would automatically mean that the blood of our Lord Jesus Christ, shed on the Cross, is of no value since we can be saved without it.
"In My Father's house are many mansions; if it were not so, I would have told you. I go to prepare a place for you" (John 14:2) those mansions are for different levels of believers each according to his level of spirituality and his works...St. Paul said it clearly we cannot be justified by works or good deeds only we have to have faith first, "knowing that a man is not justified by the works of the law but by faith in Jesus Christ, even we have believed in Christ Jesus, that we might be justified by faith in Christ and not by the works of the law; for by the works of the law no flesh shall be justified" (Gal 2:16).
An excellent book about this topic is "Salvation in the Orthodox Faith" by His Holiness Pope Shenouda.