There is a difference between assertiveness and aggressiveness. In Acts 5 there is a great example of both in one incident. In Acts 5, the apostles boldly were speaking about Jesus Christ. They were healing and helping people to see the great freedom that is available through that name.
They were assertive in speaking boldly about the truth.The reaction on the part of some of the religious leaders at that time was aggressive, they wanted to suppress and control, so they threatened them and threw them in prison.
Acts 5:17-20 (The Message)Provoked mightily by all this, the Chief Priest and those on his side, mainly the sect of Sadducees, went into action, arrested the apostles and put them in the town jail. But during the night an angel of God opened the jailhouse door and led them out. He said, “Go to the Temple and take your stand. Tell the people everything there is to say about this Life.” Promptly obedient, they entered the Temple at daybreak and went on with their teaching.
The apostles continued to speak life-giving words with boldness. They were not battling for dominion over the religious leaders, they were boldly declaring God’s love. The Sadduces and the high priest continued to move aggressively against them trying to squelch their passion, but as Gamliel said:
Acts 5:38-39:And now I say unto you, Refrain from these men, and let them alone: for if this counsel or this work be of men, it will come to nought:But if it be of God, ye cannot overthrow it; lest haply ye be found even to fight against God.
The apostles were fortified with a unique strength that only comes from God. We have to be assertive, not aggressive in boldly declaring the Word. The difference is that we speak words that build the Word not overthrow it. This is the basis of speaking the truth in love (Ephesians 4:15)