A guarantee? Maybe, maybe not. But firstly, we must understand that Christianity is not a mere name or simply a list of acts to be carried out after which you go relax. It goes way beyond that. Christianity is a way of life. It encompasses every area of your life at all times and in all places. Hence, there is no specific time to be a real Catholic and a specific time to suspend it.
Just as no one can tell who will go to Heaven, no one can tell if everyone who attends masses and confesses his sins will surely go to Heaven. But we must agree that practicing all these increases our chances of going to Heaven. For instance, a person who honestly attends masses always with the pure intention of obtaining graces from God and confesses every of his sins in deep contrition has greater possibility of attaining God’s kingdom. If such a person dies, he will most likely die in a state of grace than another who doesn’t practice all these.
“Nothing unclean will enter God’s kingdom” Revelations 21: 27 tells us better about God’s kingdom. It is true we are saved by grace but faith goes with work. So, we must make as much effort to be in a state of grace at all times which is what God requires of us.
Saint Paul in Romans 1:5, 16:26 calls real Christian Faith “the obedience of faith” which is expressed by total obedience of God through His Church. Obedience of faith results in a Christian doing all he is required to do without leaving anything out. It is disobedience to God when you deliberately skip Mass because we are God’s creation and we are obligated to worship He who is our Creator. It is also disobedience to the Church authority which commands weekly attendance.
Similarly, when you fail to confess your sins as designed by God in John 20:22-23, you are simply not sorry for your sins and have even committed a greater sin of disobedience which leaves you completely graceless.
So, all these acts, when carried out with honesty and constancy will help you live in a state of grace which puts you at a better position of going to Heaven.
Disclaimer: Prayer is not magic. God indeed answers prayers but sometimes His answer is either No or Not Now. Don't be discouraged when your prayers are not met immediately or even later. Keep trusting God who is good all the time. Thank you.