It is very clear in the Quran that God will not accept any system (Deen) other than the Islam (Submission). People who follow other systems (Deens) are referred as losers on the Day of Judgment.
3:85 And whoever follows other than submission (Islam) as a system, it will not be accepted from him, and in the Hereafter he is of the losers.
According my understanding the minimum criteria to get the salvation is mentioned in chapter 103.
103:1 By time (which is running out).
103:2 Man (Insan) is indeed in loss.
103:3 Except those who believe, and do good works, and support one another with the truth, and support one another with perseverance.
The minimum requirement for the salvation is also mentioned in the following verses.
4:124 And whoever works good, whether male or female, and is a believer, then these will be admitted to Paradise, and they will not be wronged in the least.
16:97 Whoever does good works, whether male or female, and is a believer, We will give him a good life and We will reward them their dues with the best of what they used to do.
MINIMUM CRITERIA OF SALVATION FOR JEWS, CHRISTIANS AND OTHER FAITHS
It is a misconception that people of the book such as Jews and Christians will be saved just by believing in God and the last day without practicing the Islam.
2:62 Surely those who believe; and those who are Jewish, and the Christians, and the other faiths, whoever of them believes in God and the Last Day and does good works; they will have their recompense with their Lord, and there is no fear upon them, nor will they grieve.
5:69 Surely those who believe; and those who are Jewish, and the other faiths, and the Christians, whoever of them believes in God and the Last Day and does good works; then they will have nothing to fear nor will they grieve.
Both of these verses are actually referring to the people who converted/reverted to Islam by leaving their previous faiths. So these are the people who joined the Islam (Submission) for their salvation. God has opened the door for them through these verses.
THOSE WHO BELIEVE WILL BE TESTED IN THIS LIFE BY GOD
29:2 Did the people think that they will be left to say: “We believe” without being put to the test?
29:3 We had tested those before them, so that God would distinguish those who are truthful and so that He would know the liars.