5. That those who are grafted into Christ by a true faith, and have thereby been made partakers of his life giving Spirit, are abundantly endowed with power to strive against Satan, sin, the world and their own flesh, and to win the victory; always, be it understood, with the help of the grace of the Holy Spirit, with Jesus Christ assisting them in all temptations, through his Spirit; stretching out his hand to them and (providing only that they are themselves prepared for the fight, that they entreat his aid and do not fail to help themselves) propping and upbuilding them so that by no guile or violence of Satan can they be led astray or plucked from Christ’s hands (John 10:28). But for the question whether they are not able through sloth or negligence to forsake the beginning of their life in Christ, to embrace again this present world, to depart from the holy doctrine once delivered to them, to lose their good conscience and to neglect grace, this must be the subject of more exact inquiry in the Holy Scriptures, before we can teach it with full confidence of our mind.
It's divided into two sentences. The first sentence says that believers are endowed with power to overcome the world, the flesh and the Devil, always with the grace of God, providing that they are prepared to fight, entreat His aid, and help themselves, so that Satan may not lead them astray and pluck them from His hand. The second sentence has a big BUT at the beginning and goes on to state that as for the question of whether they can through sloth or negligence forsake their faith, embrace the world, depart from the truth, and neglect grace it must be a matter of further study.
In other words, nobody can snatch us from His hand, but we can snatch ourselves. Plain as day, CTD.
And your source is? A link would be nice.