This isn't so much directed at you Cody as it is the saying that "If you believe in Jesus" so please know this isn't AT you directly.
The devil believes in Jesus...... yet we all know he is still to be cast into torment.
I used to believe it was a simple prayer that saved someone as well and once saved always saved.
However, Jesus says things such as:
Matthew 7:13-14 : Talking about "the narrow gate" but the gate to destruction is wide. I used to believe when He said that it was in reference to Him being the only way.... and a belief other than Him was the wide gate.
Then, you have things said such as this:
Matthew 7:22: Jesus speaking about people coming to him saying "Lord Lord" why would they call Him Lord, unless they believed in Him? Even saying that they did great things in His name. Yet, still He says to them "Depart from me I never knew you."
Jesus also says:
John 15:4 speaking of abiding in him and He in you, and John 15:2 speaking of branches that do not bear fruit are cut away and tossed into the fire. Meaning, IF Christ is in us, we can't help but to bear fruit..... because bearing fruit and obedience is a byproduct of Christ IN us. NOT an act that WE do, lest we boast.
Romans 11:21 speaking of if God does not spare the natural branches (Jews) then he will not spare us if we do not bear spiritual fruit.
Its NOT about a prayer of salvation, its about bearing fruit. NOT because WE bear the fruit, but because WE pick up our cross and die to our flesh daily Matthew 16:24-26. Which in doing so allows Christ to dwell IN US and in doing HIS will, we bear the fruit of obedience.
Does that mean we will EVER be perfect and not sin? No, but it DOES mean we no longer allow that sin to remain in our life and WALK in it, excusing it as "Well no bodies perfect" and standing in rebellion calling it "okay" rather than repenting of it, taking it to the Father and asking Him to cleanse your heart in obedience to strengthen and purify the heart dealing with the issue that causes the sin in the first place.
If I stub my toe in them middle of the night and curse, that is stumbling into sin.... being something came out of my mouth that is NOT fitting the character of Christ. Willingly cursing someone out just cause its Monday, that is living in sin.
The issue comes down to simply, dying to our flesh and getting to know Christ, NOT some religious prayer, not some religious tradition or simply knowing the Bible. The devil knows thew Bible and even uses it to try to temp Jesus.... yet again, we all know the devil will be tossed into the fire.
Yet so many believe that they pray the simple prayer and are saved even though they continue to live the same life they had before. But Jesus says, you do not pour new wine into old wine skins. Mark 2:22 Matthew 9:17
So if Christ's Spirit is poured into our old skin....... it will only destroy us (The lie that we are saved simply because we prayed a prayer....... which IS important, but there IS more to it than JUST that. Yes, we are absolutely saved by His grace, His mercy and His love, but NOT because of the prayer. That prayer we pray is like picking up the phone, we still have to talk to the Father, we still have to communicate to Him and take interest in Him...... that prayer is not ALL we d

therwise its like calling someone only to breathe awkwardly on the phone, saying and doing nothing.)
So the prayer is important, but its just a means of connecting to the Father, because ANY prayer we speak before that will NOT be heard. John 9:31 As before we connect to the Father in willingness to serve Him rather than the things of the world. We are wicked, no matter how "good" we think we are our hearts, as all hearts of men..... are wicked.
Im not saying this to say "You're wrong" brother. Simply putting this out there so if your heart if open and the Lord gives you the eyes to see and the ears to hear you will take this and at least think on it.
I too once believed the prayer was it as nobody told me what being a Christian entailed. So all I could do is believe the talk of the masses from people that spent "countless hours learning the Bible"
Yet the truth is, to TRULY be a follower of Christ is, to die to your flesh DAILY, die to the world DAILY, and allowing Christ to live through you. IF we remain mastered by our sins..... then Christ is not our master...... ( Matthew 6:24 Luke 16:13 ) We can not serve two masters. We either love our sin and remain IN our sin, excusing it as "Nobody is perfect" or we get down on our knees, pray in our weakness for the strength of Christ to help us CLAIM victory over that sin so that we are FREE to serve the only master a servant of Christ should have.
It might seem extreme to most people, but how extreme is it to allow your only son to be ridiculed/mocked, tempted, beaten bloody, hung on a cross...... simply to give people a chance at salvation? How extreme is revelation?
look at those that WALKED with Christ, both physical AND in the Spirit. Why is it that now we believe that Jesus is all about convenience and our comfort now? With all that the first followers of Christ endured why would WE be different?
That doesn't mean that there wont be good times and good things, but Jesus said in the parable Matthew 7:24-27 speaking of the builders. Count the cost and make sure you are okay with what it costs being a follower of Christ, if you dont then you will not be able to complete it and will only look a fool (Being told depart from me I never knew you).
Its not to say that we are saved because we obey, or that anything we do will ever be good enough to earn our salvation, its simply that our obedience, our willingness to do the will of our Father above that of our own..... these such things are examples of the spiritual fruit we bear when Christ resides in us...... NOT because we say a prayer and just move on free in our sin.