By quoting these words without their context, preachers make believe that once a person comes to Jesus, they are going to have eternal life no matter what else happens (e.g. how they may commit grievous sins).
"And I give them eternal life, and they shall never perish; neither shall anyone snatch them out of My hand." (John 10:28, NKJV)
But if we just expand our view to include the verse before, it is clear that Jesus is here speaking about people who hear him and follow him.
"My sheep hear My voice, and I know them, and they follow Me. And I give them eternal life, and they shall never perish; neither shall anyone snatch them out of My hand. " (10:27-28, NKJV)
Reading further in John's Gospel, this message of salvation for those who obey Jesus is repeated.
"As the Father loved Me, I also have loved you; abide in My love. If you keep My commandments, you will abide in My love, just as I have kept My Father’s commandments and abide in His love." (John 15:9-10, NKJV)
"This is My commandment, that you love one another as I have loved you. Greater love has no one than this, than to lay down one’s life for his friends. You are My friends if you do whatever I command you." (John 15:12-14, NKJV)