Salvation is by grace, given to us free of charge. But we need to maintain it, by preaching the gospel (not door to door specifically - tonnes of other ways eg via our live testimonies, what others can see through our lives, etc). Even if we preach out of the motive to earn salvation - that does not earn us salvation.
Matthew 7
21 "Not everyone who says to me, ‘Lord, Lord,' will enter the kingdom of heaven, but only the one who does the will of my Father who is in heaven. 22 Many will say to me on that day, ‘Lord, Lord, did we not prophesy in your name and in your name drive out demons and in your name perform many miracles?' 23 Then I will tell them plainly, ‘I never knew you. Away from me, you evildoers!'
24 "Therefore everyone who hears these words of mine and puts them into practice is like a wise man who built his house on the rock. 25 The rain came down, the streams rose, and the winds blew and beat against that house; yet it did not fall, because it had its foundation on the rock. 26 But everyone who hears these words of mine and does not put them into practice is like a foolish man who built his house on sand. 27 The rain came down, the streams rose, and the winds blew and beat against that house, and it fell with a great crash."
And what is the greatesrcommandment of God?
Mark 12
29 "The most important one," answered Jesus, "is this: ‘Hear, O Israel: The Lord our God, the Lord is one.[e] 30 Love the Lord your God with all your heart and with all your soul and with all your mind and with all your strength.'[f] 31 The second is this: ‘Love your neighbor as yourself.'[g] There is no commandment greater than these."
Look carefully at John 3:16:
16 For God so loved the world that he gave his one and only Son, that whoever believes in him shall not perish but have eternal life.
He sent His son because He LOVES first. Thus, doesn't it mean that we ought to love first before we go preach the gospel and save souls? The motive has got to be out of love and not because we want to earn salvation, or any other reasons. Other than love be the motive, those that preach the gospel will be like this mentioned in Matthew 7:21