To demonstrate that Salvation is necessary we must look at what is to come. At some time in the future the Lord will return again as shown by Mt 25:31 When the Son of man comes in his glory, and all the angels with him, then he will sit on his glorious throne. Many other passages also foretell the coming of the Lord, but this is a point that we all agree on, eliminating the need to further labor the point.
This has very real and serious consequences for us because it clearly shows that a time will come when all things are subject to the Lord. What then is the Lordís desire for us? The desire is for us to turn to him and believe in him as shown in John 6:40 For this is the will of my Father, that every one who sees the Son and believes in him should have eternal life; and I will raise him up at the last day. This desire is a great desire for the Lord. Great to the point that He sends us His angels to minister to us as shown in Hebrew 1:14 Are they (referring to angels) not all ministering spirits sent forth to serve, for the sake of those who are to obtain salvation?
So far it has been demonstrated that there is a need for Salvation, and that the Lord desires us to achieve Salvation; but not that Salvation is necessary. The necessity of Salvation is demonstrated by the fact that eternal reward or punishment is possible. God will judge us at the end of time as shown in Romans 2:6-8 For he will render to every man according to his works: to those who by patience in well-doing seek for glory and honor and immortality, he will give eternal life; but for those who are factious and do not obey the truth, but obey wickedness, there will be wrath and fury. This judgment can lead to either reward or punishment as shown in Mk 16:15-16 Go into the whole world and proclaim the good news to all creation. The man who believes it and accepts baptism will be saved; the man who refuses to believe in it will be condemned.
Yes, Salvation is necessary. At the return of the Lord we will either be rewarded with Salvation or sent to eternal punishment for wicked deeds. We have the means for Salvation as shown in Romans 1:16 For I am not ashamed of the gospel: it is the power of God for salvation to every one who has faith, to the Jew first and also to the Greek. But the choice is ours. As shown in John 3:36 He who believes in the Son has eternal life; he who does not obey the Son shall not see life, but the wrath of God rests upon him we do have the need to seek out Salvation.