1 Cor CHAP. XV. 3 By Christes resurrection, 12 he proueth the necessitie of our resurrection, against all such as deny the resurrection of the body. 21 The fruit, 35 and maner thereof, 51 And of the changing of them, that shall bee found aliue at the last day.
1 Moreouer brethren, I declare vnto you the Gospel which I preached vnto you, which also you haue receiued, and wherein yee stand. 2 By which also yee are saued, if yee keepe in memorie what I preached vnto you, vnlesse yee haue beleeued in vaine. 3 For I deliuered vnto you first of all, that which I also receiued, how that Christ died for our sinnes according to the Scriptures: 4 And that he was buried, and that he rose againe the third day according to the Scriptures. 5 And that he was seene of Cephas, then of the twelue. 6 And that hee was seene of aboue fiue hundred brethren at once: of whom the greater part remaine vnto this present, but some are fallen asleepe. 7 After that, he was seen of Iames, then of all the Apostles. 8 And last of all he was seene of me also, as of one borne out of due time. 9 For I am the least of the Apostles, that am not meet to be called an Apostle because I persecuted þe Church of God. 10 But by the grace of God I am what I am: and his grace which was bestowed vpō me, was not in vaine: But I laboured more abundantly then they all, yet not I, but the grace of God which was with me: 11 Therefore, whether it were I or they, so we preach, aud so ye beleeued. 12 Now if Christ be preached that he rose from the dead, how say some among you, that there is no resurrection of the dead? 13 But if there be no resurrection of the dead, then is Christ not risen. 14 And if Christ be not risen, then is our preaching vaine, and your faith is also vaine: 15 Yea, and we are found false witnesses of God, because we haue testified of God, that he raised vp Christ: whom hee raised not vp, if so bee that the dead rise not. 16 For if the dead rise not, then is not Christ raised. 17 And if Christ be not raised, your faith is vaine, ye are yet in your sinnes. 18 Then they also which are fallen asleepe in Christ, are perished. 19 If in this life only we haue hope in Christ, wee are of all men most miserable. 20 But now is Christ risen from the dead, and become the first fruits of them that slept.
As you can see  it is possible to believe in vaine and it is explained further down  that someone has believed in vaine if Christ is not risen. As you can see in  there is an if clause. We are to keep in memory the gospel to be eternally saved. This is the seed in the good ground. The word of the kingdom in Matthew 7:19 is The Gospel.
The gospel of your salvation, 1 Corinthians 15:3-4.